[PATCH v2 0/7] mm: introduce memfd_secret system call to create "secret" memory areas

From: Mike Rapoport <redacted>

Hi,

This is an implementation of "secret" mappings backed by a file descriptor. 

v2 changes:
* Follow Michael's suggestion and name the new system call 'memfd_secret'
* Add kernel-parameters documentation about the boot option
* Fix i386-tinyconfig regression reported by the kbuild bot.
  CONFIG_SECRETMEM now depends on !EMBEDDED to disable it on small systems
  from one side and still make it available unconditionally on
  architectures that support SET_DIRECT_MAP.

The file descriptor backing secret memory mappings is created using a
dedicated memfd_secret system call The desired protection mode for the
memory is configured using flags parameter of the system call. The mmap()
of the file descriptor created with memfd_secret() will create a "secret"
memory mapping. The pages in that mapping will be marked as not present in
the direct map and will have desired protection bits set in the user page
table. For instance, current implementation allows uncached mappings.

Although normally Linux userspace mappings are protected from other users, 
such secret mappings are useful for environments where a hostile tenant is
trying to trick the kernel into giving them access to other tenants
mappings.

Additionally, the secret mappings may be used as a mean to protect guest
memory in a virtual machine host.

For demonstration of secret memory usage we've created a userspace library
[1] that does two things: the first is act as a preloader for openssl to
redirect all the OPENSSL_malloc calls to secret memory meaning any secret
keys get automatically protected this way and the other thing it does is
expose the API to the user who needs it. We anticipate that a lot of the
use cases would be like the openssl one: many toolkits that deal with
secret keys already have special handling for the memory to try to give
them greater protection, so this would simply be pluggable into the
toolkits without any need for user application modification.

I've hesitated whether to continue to use new flags to memfd_create() or to
add a new system call and I've decided to use a new system call after I've
started to look into man pages update. There would have been two completely
independent descriptions and I think it would have been very confusing.

Hiding secret memory mappings behind an anonymous file allows (ab)use of
the page cache for tracking pages allocated for the "secret" mappings as
well as using address_space_operations for e.g. page migration callbacks.

The anonymous file may be also used implicitly, like hugetlb files, to
implement mmap(MAP_SECRET) and use the secret memory areas with "native" mm
ABIs in the future.

As the fragmentation of the direct map was one of the major concerns raised
during the previous postings, I've added an amortizing cache of PMD-size
pages to each file descriptor and an ability to reserve large chunks of the
physical memory at boot time and then use this memory as an allocation pool
for the secret memory areas.

v1: https://lore.kernel.org/lkml/20200720092435.17469-1-rppt@kernel.org/
rfc-v2: https://lore.kernel.org/lkml/20200706172051.19465-1-rppt@kernel.org/
rfc-v1: https://lore.kernel.org/lkml/20200130162340.GA14232@rapoport-lnx/

Mike Rapoport (7):
  mm: add definition of PMD_PAGE_ORDER
  mmap: make mlock_future_check() global
  mm: introduce memfd_secret system call to create "secret" memory areas
  arch, mm: wire up memfd_secret system call were relevant
  mm: secretmem: use PMD-size pages to amortize direct map fragmentation
  mm: secretmem: add ability to reserve memory at boot
  mm: secretmem: add ability to reserve memory at boot

 .../admin-guide/kernel-parameters.txt         |   4 +
 arch/arm64/include/asm/unistd32.h             |   2 +
 arch/arm64/include/uapi/asm/unistd.h          |   1 +
 arch/riscv/include/asm/unistd.h               |   1 +
 arch/x86/entry/syscalls/syscall_32.tbl        |   1 +
 arch/x86/entry/syscalls/syscall_64.tbl        |   1 +
 fs/dax.c                                      |  10 +-
 include/linux/pgtable.h                       |   3 +
 include/linux/syscalls.h                      |   1 +
 include/uapi/asm-generic/unistd.h             |   7 +-
 include/uapi/linux/magic.h                    |   1 +
 include/uapi/linux/secretmem.h                |   9 +
 kernel/sys_ni.c                               |   2 +
 mm/Kconfig                                    |   4 +
 mm/Makefile                                   |   1 +
 mm/internal.h                                 |   3 +
 mm/mmap.c                                     |   5 +-
 mm/secretmem.c                                | 453 ++++++++++++++++++
 18 files changed, 500 insertions(+), 9 deletions(-)
 create mode 100644 include/uapi/linux/secretmem.h
 create mode 100644 mm/secretmem.c

-- 
2.26.2

From: Mike Rapoport <redacted>

The definition of PMD_PAGE_ORDER denoting the number of base pages in the
second-level leaf page is already used by DAX and maybe handy in other
cases as well.

Several architectures already have definition of PMD_ORDER as the size of
second level page table, so to avoid conflict with these definitions use
PMD_PAGE_ORDER name and update DAX respectively.

Signed-off-by: Mike Rapoport <redacted>
---
 fs/dax.c                | 10 +++++-----
 include/linux/pgtable.h |  3 +++
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 11b16729b86f..b91d8c8dda45 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -50,7 +50,7 @@ static inline unsigned int pe_order(enum page_entry_size pe_size)
 #define PG_PMD_NR	(PMD_SIZE >> PAGE_SHIFT)
 
 /* The order of a PMD entry */
-#define PMD_ORDER	(PMD_SHIFT - PAGE_SHIFT)
+#define PMD_PAGE_ORDER	(PMD_SHIFT - PAGE_SHIFT)
 
 static wait_queue_head_t wait_table[DAX_WAIT_TABLE_ENTRIES];
 

@@ -98,7 +98,7 @@ static bool dax_is_locked(void *entry)
 static unsigned int dax_entry_order(void *entry)
 {
 	if (xa_to_value(entry) & DAX_PMD)
-		return PMD_ORDER;
+		return PMD_PAGE_ORDER;
 	return 0;
 }
 

@@ -1456,7 +1456,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 {
 	struct vm_area_struct *vma = vmf->vma;
 	struct address_space *mapping = vma->vm_file->f_mapping;
-	XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, PMD_ORDER);
+	XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, PMD_PAGE_ORDER);
 	unsigned long pmd_addr = vmf->address & PMD_MASK;
 	bool write = vmf->flags & FAULT_FLAG_WRITE;
 	bool sync;

@@ -1515,7 +1515,7 @@ static vm_fault_t dax_iomap_pmd_fault(struct vm_fault *vmf, pfn_t *pfnp,
 	 * entry is already in the array, for instance), it will return
 	 * VM_FAULT_FALLBACK.
 	 */
-	entry = grab_mapping_entry(&xas, mapping, PMD_ORDER);
+	entry = grab_mapping_entry(&xas, mapping, PMD_PAGE_ORDER);
 	if (xa_is_internal(entry)) {
 		result = xa_to_internal(entry);
 		goto fallback;

@@ -1681,7 +1681,7 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
 	if (order == 0)
 		ret = vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
 #ifdef CONFIG_FS_DAX_PMD
-	else if (order == PMD_ORDER)
+	else if (order == PMD_PAGE_ORDER)
 		ret = vmf_insert_pfn_pmd(vmf, pfn, FAULT_FLAG_WRITE);
 #endif
 	else

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 56c1e8eb7bb0..79f8443609e7 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h

@@ -28,6 +28,9 @@
 #define USER_PGTABLES_CEILING	0UL
 #endif
 
+/* Number of base pages in a second level leaf page */
+#define PMD_PAGE_ORDER	(PMD_SHIFT - PAGE_SHIFT)
+
 /*
  * A page table page can be thought of an array like this: pXd_t[PTRS_PER_PxD]
  *

-- 
2.26.2

From: Mike Rapoport <redacted>

It will be used by the upcoming secret memory implementation.

Signed-off-by: Mike Rapoport <redacted>
---
 mm/internal.h | 3 +++
 mm/mmap.c     | 5 ++---
 2 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index 9886db20d94f..af0a92f8f6bc 100644
--- a/mm/internal.h
+++ b/mm/internal.h

@@ -349,6 +349,9 @@ static inline void munlock_vma_pages_all(struct vm_area_struct *vma)
 extern void mlock_vma_page(struct page *page);
 extern unsigned int munlock_vma_page(struct page *page);
 
+extern int mlock_future_check(struct mm_struct *mm, unsigned long flags,
+			      unsigned long len);
+
 /*
  * Clear the page's PageMlocked().  This can be useful in a situation where
  * we want to unconditionally remove a page from the pagecache -- e.g.,

diff --git a/mm/mmap.c b/mm/mmap.c
index 8c7ca737a19b..ee92b7b4b185 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -1310,9 +1310,8 @@ static inline unsigned long round_hint_to_min(unsigned long hint)
 	return hint;
 }
 
-static inline int mlock_future_check(struct mm_struct *mm,
-				     unsigned long flags,
-				     unsigned long len)
+int mlock_future_check(struct mm_struct *mm, unsigned long flags,
+		       unsigned long len)
 {
 	unsigned long locked, lock_limit;
 

-- 
2.26.2

From: Mike Rapoport <redacted>

Introduce "memfd_secret" system call with the ability to create memory
areas visible only in the context of the owning process and not mapped not
only to other processes but in the kernel page tables as well.

The user will create a file descriptor using the memfd_secret() system call
where flags supplied as a parameter to this system call will define the
desired protection mode for the memory associated with that file
descriptor. Currently there are two protection modes:

* exclusive - the memory area is unmapped from the kernel direct map and it
              is present only in the page tables of the owning mm.
* uncached  - the memory area is present only in the page tables of the
              owning mm and it is mapped there as uncached.

For instance, the following example will create an uncached mapping (error
handling is omitted):

	fd = memfd_secret(SECRETMEM_UNCACHED);
	ftruncate(fd, MAP_SIZE);
	ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

Signed-off-by: Mike Rapoport <redacted>
---
 include/uapi/linux/magic.h     |   1 +
 include/uapi/linux/secretmem.h |   9 ++
 kernel/sys_ni.c                |   2 +
 mm/Kconfig                     |   4 +
 mm/Makefile                    |   1 +
 mm/secretmem.c                 | 266 +++++++++++++++++++++++++++++++++
 6 files changed, 283 insertions(+)
 create mode 100644 include/uapi/linux/secretmem.h
 create mode 100644 mm/secretmem.c

diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h
index f3956fc11de6..35687dcb1a42 100644
--- a/include/uapi/linux/magic.h
+++ b/include/uapi/linux/magic.h

@@ -97,5 +97,6 @@
 #define DEVMEM_MAGIC		0x454d444d	/* "DMEM" */
 #define Z3FOLD_MAGIC		0x33
 #define PPC_CMM_MAGIC		0xc7571590
+#define SECRETMEM_MAGIC		0x5345434d	/* "SECM" */
 
 #endif /* __LINUX_MAGIC_H__ */

diff --git a/include/uapi/linux/secretmem.h b/include/uapi/linux/secretmem.h
new file mode 100644
index 000000000000..cef7a59f7492
--- /dev/null
+++ b/include/uapi/linux/secretmem.h

@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_SECRERTMEM_H
+#define _UAPI_LINUX_SECRERTMEM_H
+
+/* secretmem operation modes */
+#define SECRETMEM_EXCLUSIVE	0x1
+#define SECRETMEM_UNCACHED	0x2
+
+#endif /* _UAPI_LINUX_SECRERTMEM_H */

diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 3b69a560a7ac..fd40e1c083e5 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c

@@ -349,6 +349,8 @@ COND_SYSCALL(pkey_mprotect);
 COND_SYSCALL(pkey_alloc);
 COND_SYSCALL(pkey_free);
 
+/* memfd_secret */
+COND_SYSCALL(memfd_secret);
 
 /*
  * Architecture specific weak syscall entries.

diff --git a/mm/Kconfig b/mm/Kconfig
index f2104cc0d35c..8378175e72a4 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig

@@ -872,4 +872,8 @@ config ARCH_HAS_HUGEPD
 config MAPPING_DIRTY_HELPERS
         bool
 
+config SECRETMEM
+        def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED
+	select GENERIC_ALLOCATOR
+
 endmenu

diff --git a/mm/Makefile b/mm/Makefile
index 6e9d46b2efc9..c2aa7a393b73 100644
--- a/mm/Makefile
+++ b/mm/Makefile

@@ -121,3 +121,4 @@ obj-$(CONFIG_MEMFD_CREATE) += memfd.o
 obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o
 obj-$(CONFIG_PTDUMP_CORE) += ptdump.o
 obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o
+obj-$(CONFIG_SECRETMEM) += secretmem.o

diff --git a/mm/secretmem.c b/mm/secretmem.c
new file mode 100644
index 000000000000..9d29f3e1c49d
--- /dev/null
+++ b/mm/secretmem.c

@@ -0,0 +1,266 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/mount.h>
+#include <linux/memfd.h>
+#include <linux/bitops.h>
+#include <linux/printk.h>
+#include <linux/pagemap.h>
+#include <linux/syscalls.h>
+#include <linux/pseudo_fs.h>
+#include <linux/set_memory.h>
+#include <linux/sched/signal.h>
+
+#include <uapi/linux/secretmem.h>
+#include <uapi/linux/magic.h>
+
+#include <asm/tlbflush.h>
+
+#include "internal.h"
+
+#undef pr_fmt
+#define pr_fmt(fmt) "secretmem: " fmt
+
+#define SECRETMEM_MODE_MASK	(SECRETMEM_EXCLUSIVE | SECRETMEM_UNCACHED)
+#define SECRETMEM_FLAGS_MASK	SECRETMEM_MODE_MASK
+
+struct secretmem_ctx {
+	unsigned int mode;
+};
+
+static struct page *secretmem_alloc_page(gfp_t gfp)
+{
+	/*
+	 * FIXME: use a cache of large pages to reduce the direct map
+	 * fragmentation
+	 */
+	return alloc_page(gfp);
+}
+
+static vm_fault_t secretmem_fault(struct vm_fault *vmf)
+{
+	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	pgoff_t offset = vmf->pgoff;
+	unsigned long addr;
+	struct page *page;
+	int ret = 0;
+
+	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+		return vmf_error(-EINVAL);
+
+	page = find_get_entry(mapping, offset);
+	if (!page) {
+		page = secretmem_alloc_page(vmf->gfp_mask);
+		if (!page)
+			return vmf_error(-ENOMEM);
+
+		ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
+		if (unlikely(ret))
+			goto err_put_page;
+
+		ret = set_direct_map_invalid_noflush(page);
+		if (ret)
+			goto err_del_page_cache;
+
+		addr = (unsigned long)page_address(page);
+		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+		__SetPageUptodate(page);
+
+		ret = VM_FAULT_LOCKED;
+	}
+
+	vmf->page = page;
+	return ret;
+
+err_del_page_cache:
+	delete_from_page_cache(page);
+err_put_page:
+	put_page(page);
+	return vmf_error(ret);
+}
+
+static const struct vm_operations_struct secretmem_vm_ops = {
+	.fault = secretmem_fault,
+};
+
+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}
+
+const struct file_operations secretmem_fops = {
+	.mmap		= secretmem_mmap,
+};
+
+static bool secretmem_isolate_page(struct page *page, isolate_mode_t mode)
+{
+	return false;
+}
+
+static int secretmem_migratepage(struct address_space *mapping,
+				 struct page *newpage, struct page *page,
+				 enum migrate_mode mode)
+{
+	return -EBUSY;
+}
+
+static void secretmem_freepage(struct page *page)
+{
+	set_direct_map_default_noflush(page);
+}
+
+static const struct address_space_operations secretmem_aops = {
+	.freepage	= secretmem_freepage,
+	.migratepage	= secretmem_migratepage,
+	.isolate_page	= secretmem_isolate_page,
+};
+
+static struct vfsmount *secretmem_mnt;
+
+static struct file *secretmem_file_create(unsigned long flags)
+{
+	struct file *file = ERR_PTR(-ENOMEM);
+	struct secretmem_ctx *ctx;
+	struct inode *inode;
+
+	inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
+	if (IS_ERR(inode))
+		return ERR_CAST(inode);
+
+	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
+	if (!ctx)
+		goto err_free_inode;
+
+	file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
+				 O_RDWR, &secretmem_fops);
+	if (IS_ERR(file))
+		goto err_free_ctx;
+
+	mapping_set_unevictable(inode->i_mapping);
+
+	inode->i_mapping->private_data = ctx;
+	inode->i_mapping->a_ops = &secretmem_aops;
+
+	/* pretend we are a normal file with zero size */
+	inode->i_mode |= S_IFREG;
+	inode->i_size = 0;
+
+	file->private_data = ctx;
+
+	ctx->mode = flags & SECRETMEM_MODE_MASK;
+
+	return file;
+
+err_free_ctx:
+	kfree(ctx);
+err_free_inode:
+	iput(inode);
+	return file;
+}
+
+SYSCALL_DEFINE1(memfd_secret, unsigned long, flags)
+{
+	struct file *file;
+	unsigned int mode;
+	int fd, err;
+
+	/* make sure local flags do not confict with global fcntl.h */
+	BUILD_BUG_ON(SECRETMEM_FLAGS_MASK & O_CLOEXEC);
+
+	if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
+		return -EINVAL;
+
+	/* modes are mutually exclusive, only one mode bit should be set */
+	mode = flags & SECRETMEM_FLAGS_MASK;
+	if (ffs(mode) != fls(mode))
+		return -EINVAL;
+
+	fd = get_unused_fd_flags(flags & O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	file = secretmem_file_create(flags);
+	if (IS_ERR(file)) {
+		err = PTR_ERR(file);
+		goto err_put_fd;
+	}
+
+	file->f_flags |= O_LARGEFILE;
+
+	fd_install(fd, file);
+	return fd;
+
+err_put_fd:
+	put_unused_fd(fd);
+	return err;
+}
+
+static void secretmem_evict_inode(struct inode *inode)
+{
+	struct secretmem_ctx *ctx = inode->i_private;
+
+	truncate_inode_pages_final(&inode->i_data);
+	clear_inode(inode);
+	kfree(ctx);
+}
+
+static const struct super_operations secretmem_super_ops = {
+	.evict_inode = secretmem_evict_inode,
+};
+
+static int secretmem_init_fs_context(struct fs_context *fc)
+{
+	struct pseudo_fs_context *ctx = init_pseudo(fc, SECRETMEM_MAGIC);
+
+	if (!ctx)
+		return -ENOMEM;
+	ctx->ops = &secretmem_super_ops;
+
+	return 0;
+}
+
+static struct file_system_type secretmem_fs = {
+	.name		= "secretmem",
+	.init_fs_context = secretmem_init_fs_context,
+	.kill_sb	= kill_anon_super,
+};
+
+static int secretmem_init(void)
+{
+	int ret = 0;
+
+	secretmem_mnt = kern_mount(&secretmem_fs);
+	if (IS_ERR(secretmem_mnt))
+		ret = PTR_ERR(secretmem_mnt);
+
+	return ret;
+}
+fs_initcall(secretmem_init);

-- 
2.26.2

Hi Mike,

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

For instance, the following example will create an uncached mapping (error
handling is omitted):

	fd = memfd_secret(SECRETMEM_UNCACHED);
	ftruncate(fd, MAP_SIZE);
	ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

[...]

+static struct page *secretmem_alloc_page(gfp_t gfp)
+{
+	/*
+	 * FIXME: use a cache of large pages to reduce the direct map
+	 * fragmentation
+	 */
+	return alloc_page(gfp);
+}
+
+static vm_fault_t secretmem_fault(struct vm_fault *vmf)
+{
+	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	pgoff_t offset = vmf->pgoff;
+	unsigned long addr;
+	struct page *page;
+	int ret = 0;
+
+	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+		return vmf_error(-EINVAL);
+
+	page = find_get_entry(mapping, offset);
+	if (!page) {
+		page = secretmem_alloc_page(vmf->gfp_mask);
+		if (!page)
+			return vmf_error(-ENOMEM);
+
+		ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
+		if (unlikely(ret))
+			goto err_put_page;
+
+		ret = set_direct_map_invalid_noflush(page);
+		if (ret)
+			goto err_del_page_cache;
+
+		addr = (unsigned long)page_address(page);
+		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+		__SetPageUptodate(page);
+
+		ret = VM_FAULT_LOCKED;
+	}
+
+	vmf->page = page;
+	return ret;
+
+err_del_page_cache:
+	delete_from_page_cache(page);
+err_put_page:
+	put_page(page);
+	return vmf_error(ret);
+}
+
+static const struct vm_operations_struct secretmem_vm_ops = {
+	.fault = secretmem_fault,
+};
+
+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

-- 
Catalin

On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:

Hi Mike,

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

quoted

For instance, the following example will create an uncached mapping (error
handling is omitted):

	fd = memfd_secret(SECRETMEM_UNCACHED);
	ftruncate(fd, MAP_SIZE);
	ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

[...]

quoted

+static struct page *secretmem_alloc_page(gfp_t gfp)
+{
+	/*
+	 * FIXME: use a cache of large pages to reduce the direct map
+	 * fragmentation
+	 */
+	return alloc_page(gfp);
+}
+
+static vm_fault_t secretmem_fault(struct vm_fault *vmf)
+{
+	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
+	struct inode *inode = file_inode(vmf->vma->vm_file);
+	pgoff_t offset = vmf->pgoff;
+	unsigned long addr;
+	struct page *page;
+	int ret = 0;
+
+	if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode))
+		return vmf_error(-EINVAL);
+
+	page = find_get_entry(mapping, offset);
+	if (!page) {
+		page = secretmem_alloc_page(vmf->gfp_mask);
+		if (!page)
+			return vmf_error(-ENOMEM);
+
+		ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask);
+		if (unlikely(ret))
+			goto err_put_page;
+
+		ret = set_direct_map_invalid_noflush(page);
+		if (ret)
+			goto err_del_page_cache;
+
+		addr = (unsigned long)page_address(page);
+		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+		__SetPageUptodate(page);
+
+		ret = VM_FAULT_LOCKED;
+	}
+
+	vmf->page = page;
+	return ret;
+
+err_del_page_cache:
+	delete_from_page_cache(page);
+err_put_page:
+	put_page(page);
+	return vmf_error(ret);
+}
+
+static const struct vm_operations_struct secretmem_vm_ops = {
+	.fault = secretmem_fault,
+};
+
+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

 
Hmm, on x86 it's also Strongly Ordered, but I didn't find any alignment
restrictions. Is there a mode for arm64 that can provide similar
semantics?

Would it make sence to use something like

#define pgprot_uncached(prot) \
	__pgprot_modify(prot, PTE_ATTRINDX_MASK, \
			PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN)

or is it too weak?

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

Well, the idea of uncached mappings came from Elena [1] to prevent
possibility of side channels that leak user space memory. So I think
even without cache flushing after the allocation, user space is
protected as all its memory accesses bypass cache so even after the page
is freed there won't be stale data in the cache.

I think that it makes sense to limit SECRETMEM_UNCACHED only for
architectures that define an appropriate protection, e.g.
pgprot_uncahced(). For x86 it can be aliased to pgprot_noncached() and
other architecures can define their versions.

[1] https://lore.kernel.org/lkml/2236FBA76BA1254E88B949DDB74E612BA4EEC0CE@IRSMSX102.ger.corp.intel.com/

-- 
Sincerely yours,
Mike.

On Thu, Jul 30, 2020 at 11:44:09PM +0300, Mike Rapoport wrote:

On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:

quoted

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

quoted

For instance, the following example will create an uncached mapping (error
handling is omitted):

	fd = memfd_secret(SECRETMEM_UNCACHED);
	ftruncate(fd, MAP_SIZE);
	ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

[...]

quoted

quoted

+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

 
Hmm, on x86 it's also Strongly Ordered, but I didn't find any alignment
restrictions. Is there a mode for arm64 that can provide similar
semantics?

Would it make sence to use something like

#define pgprot_uncached(prot) \
	__pgprot_modify(prot, PTE_ATTRINDX_MASK, \
			PTE_ATTRINDX(MT_NORMAL_NC) | PTE_PXN)

or is it too weak?

Reading Elena's email, that's about preventing speculative loads. While
the arm64 Normal NC is non-cacheable (equivalent to write-combine), a
CPU is allowed to speculatively read from it. A carefully crafted gadget
could leave an imprint on a different part of the cache via speculative
execution based on a value in the secret memory. So IIUC, we want memory
that cannot be speculatively loaded from and that would be Device memory
on arm64 (with the alignment restrictions).

Now, I think we could relax this to Device_GRE. So maybe add a
pgprot_nospec() and allow architectures to define whatever they find
suitable. The exact semantics will be different between architectures.

quoted

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

Well, the idea of uncached mappings came from Elena [1] to prevent
possibility of side channels that leak user space memory. So I think
even without cache flushing after the allocation, user space is
protected as all its memory accesses bypass cache so even after the page
is freed there won't be stale data in the cache.

I think that it makes sense to limit SECRETMEM_UNCACHED only for
architectures that define an appropriate protection, e.g.
pgprot_uncahced(). For x86 it can be aliased to pgprot_noncached() and
other architecures can define their versions.

Indeed, though as I said above, maybe use a name that suggests no
speculation since non-cacheable doesn't always guarantee that. Something
like pgprot_nospec() and SECRETMEM_NOSPEC.

However, your implementation still has the problem that such memory must
have the caches flushed before being mapped in user-space, otherwise we
leak other secrets via such pages to the caller. The only generic API we
have in the kernel for such things is the DMA one. If hch doesn't mind,
you could abuse it and call arch_dma_prep_coherent() prior to
set_direct_map_invalid_noflush() (if the mapping is non-cacheable).

-- 
Catalin

On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

quoted

+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

It's also worth noting that in a virtual machine this is liable to be
either broken (with a potential loss of coherency if the host has a
cacheable alias as existing KVM hosts have), or pointless (if the host
uses S2FWB to upgrade Stage-1 attribues to cacheable as existing KVM
hosts also have).

I think that trying to avoid the data caches creates many more problems
than it solves, and I don't think there's a strong justification for
trying to support that on arm64 to begin with, so I'd rather entirely
opt-out on supporting SECRETMEM_UNCACHED.

Thanks,
Mark.

On Fri, Jul 31, 2020 at 03:29:05PM +0100, Mark Rutland wrote:

On Thu, Jul 30, 2020 at 05:22:10PM +0100, Catalin Marinas wrote:

quoted

On Mon, Jul 27, 2020 at 07:29:31PM +0300, Mike Rapoport wrote:

quoted

+static int secretmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	struct secretmem_ctx *ctx = file->private_data;
+	unsigned long mode = ctx->mode;
+	unsigned long len = vma->vm_end - vma->vm_start;
+
+	if (!mode)
+		return -EINVAL;
+
+	if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
+		return -EINVAL;
+
+	if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len))
+		return -EAGAIN;
+
+	switch (mode) {
+	case SECRETMEM_UNCACHED:
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+		fallthrough;
+	case SECRETMEM_EXCLUSIVE:
+		vma->vm_ops = &secretmem_vm_ops;
+		break;
+	default:
+		return -EINVAL;
+	}
+
+	vma->vm_flags |= VM_LOCKED;
+
+	return 0;
+}

I think the uncached mapping is not the right thing for arm/arm64. First
of all, pgprot_noncached() gives us Strongly Ordered (Device memory)
semantics together with not allowing unaligned accesses. I suspect the
semantics are different on x86.

quoted

The second, more serious problem, is that I can't find any place where
the caches are flushed for the page mapped on fault. When a page is
allocated, assuming GFP_ZERO, only the caches are guaranteed to be
zeroed. Exposing this subsequently to user space as uncached would allow
the user to read stale data prior to zeroing. The arm64
set_direct_map_default_noflush() doesn't do any cache maintenance.

It's also worth noting that in a virtual machine this is liable to be
either broken (with a potential loss of coherency if the host has a
cacheable alias as existing KVM hosts have), or pointless (if the host
uses S2FWB to upgrade Stage-1 attribues to cacheable as existing KVM
hosts also have).

I think that trying to avoid the data caches creates many more problems
than it solves, and I don't think there's a strong justification for
trying to support that on arm64 to begin with, so I'd rather entirely
opt-out on supporting SECRETMEM_UNCACHED.

Good point, I forgot the virtualisation aspect. So unless there is a
hypervisor API to unmap it from the host memory, the uncached option
isn't of much use on arm64.

-- 
Catalin

From: Mike Rapoport <redacted>

Wire up memfd_secret system call on architectures that define
ARCH_HAS_SET_DIRECT_MAP, namely arm64, risc-v and x86.

Signed-off-by: Mike Rapoport <redacted>
Acked-by: Palmer Dabbelt <redacted>
---
 arch/arm64/include/asm/unistd32.h      | 2 ++
 arch/arm64/include/uapi/asm/unistd.h   | 1 +
 arch/riscv/include/asm/unistd.h        | 1 +
 arch/x86/entry/syscalls/syscall_32.tbl | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl | 1 +
 include/linux/syscalls.h               | 1 +
 include/uapi/asm-generic/unistd.h      | 7 ++++++-
 7 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/unistd32.h b/arch/arm64/include/asm/unistd32.h
index 6d95d0c8bf2f..a379ba31f7c4 100644
--- a/arch/arm64/include/asm/unistd32.h
+++ b/arch/arm64/include/asm/unistd32.h

@@ -885,6 +885,8 @@ __SYSCALL(__NR_openat2, sys_openat2)
 __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
 #define __NR_faccessat2 439
 __SYSCALL(__NR_faccessat2, sys_faccessat2)
+#define __NR_memfd_secret 439
+__SYSCALL(__NR_memfd_secret, sys_memfd_secret)
 
 /*
  * Please add new compat syscalls above this comment and update

diff --git a/arch/arm64/include/uapi/asm/unistd.h b/arch/arm64/include/uapi/asm/unistd.h
index f83a70e07df8..ce2ee8f1e361 100644
--- a/arch/arm64/include/uapi/asm/unistd.h
+++ b/arch/arm64/include/uapi/asm/unistd.h

@@ -20,5 +20,6 @@
 #define __ARCH_WANT_SET_GET_RLIMIT
 #define __ARCH_WANT_TIME32_SYSCALLS
 #define __ARCH_WANT_SYS_CLONE3
+#define __ARCH_WANT_MEMFD_SECRET
 
 #include <asm-generic/unistd.h>

diff --git a/arch/riscv/include/asm/unistd.h b/arch/riscv/include/asm/unistd.h
index 977ee6181dab..6c316093a1e5 100644
--- a/arch/riscv/include/asm/unistd.h
+++ b/arch/riscv/include/asm/unistd.h

@@ -9,6 +9,7 @@
  */
 
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_MEMFD_SECRET
 
 #include <uapi/asm/unistd.h>
 

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index d8f8a1a69ed1..6f8b5978053b 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

@@ -443,3 +443,4 @@
 437	i386	openat2			sys_openat2
 438	i386	pidfd_getfd		sys_pidfd_getfd
 439	i386	faccessat2		sys_faccessat2
+440	i386	memfd_secret		sys_memfd_secret

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 78847b32e137..7d3775d1c3d7 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl

@@ -360,6 +360,7 @@
 437	common	openat2			sys_openat2
 438	common	pidfd_getfd		sys_pidfd_getfd
 439	common	faccessat2		sys_faccessat2
+440	common	memfd_secret		sys_memfd_secret
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index b951a87da987..e4d7b30867c6 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h

@@ -1005,6 +1005,7 @@ asmlinkage long sys_pidfd_send_signal(int pidfd, int sig,
 				       siginfo_t __user *info,
 				       unsigned int flags);
 asmlinkage long sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
+asmlinkage long sys_memfd_secret(unsigned long flags);
 
 /*
  * Architecture-specific system calls

diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index f4a01305d9a6..7b288347c5a9 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h

@@ -858,8 +858,13 @@ __SYSCALL(__NR_pidfd_getfd, sys_pidfd_getfd)
 #define __NR_faccessat2 439
 __SYSCALL(__NR_faccessat2, sys_faccessat2)
 
+#ifdef __ARCH_WANT_MEMFD_SECRET
+#define __NR_memfd_secret 440
+__SYSCALL(__NR_memfd_secret, sys_memfd_secret)
+#endif
+
 #undef __NR_syscalls
-#define __NR_syscalls 440
+#define __NR_syscalls 441
 
 /*
  * 32 bit systems traditionally used different

-- 
2.26.2

On Mon, Jul 27, 2020 at 6:30 PM Mike Rapoport [off-list ref] wrote:

From: Mike Rapoport <redacted>

Wire up memfd_secret system call on architectures that define
ARCH_HAS_SET_DIRECT_MAP, namely arm64, risc-v and x86.

Signed-off-by: Mike Rapoport <redacted>
Acked-by: Palmer Dabbelt <redacted>

Acked-by: Arnd Bergmann <arnd@arndb.de>

From: Mike Rapoport <redacted>

Removing a PAGE_SIZE page from the direct map every time such page is
allocated for a secret memory mapping will cause severe fragmentation of
the direct map. This fragmentation can be reduced by using PMD-size pages
as a pool for small pages for secret memory mappings.

Add a gen_pool per secretmem inode and lazily populate this pool with
PMD-size pages.

Signed-off-by: Mike Rapoport <redacted>
---
 mm/secretmem.c | 107 ++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 88 insertions(+), 19 deletions(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index 9d29f3e1c49d..da609701e10e 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c

@@ -6,6 +6,7 @@
 #include <linux/bitops.h>
 #include <linux/printk.h>
 #include <linux/pagemap.h>
+#include <linux/genalloc.h>
 #include <linux/syscalls.h>
 #include <linux/pseudo_fs.h>
 #include <linux/set_memory.h>

@@ -25,24 +26,66 @@
 #define SECRETMEM_FLAGS_MASK	SECRETMEM_MODE_MASK
 
 struct secretmem_ctx {
+	struct gen_pool *pool;
 	unsigned int mode;
 };
 
-static struct page *secretmem_alloc_page(gfp_t gfp)
+static int secretmem_pool_increase(struct secretmem_ctx *ctx, gfp_t gfp)
 {
-	/*
-	 * FIXME: use a cache of large pages to reduce the direct map
-	 * fragmentation
-	 */
-	return alloc_page(gfp);
+	unsigned long nr_pages = (1 << PMD_PAGE_ORDER);
+	struct gen_pool *pool = ctx->pool;
+	unsigned long addr;
+	struct page *page;
+	int err;
+
+	page = alloc_pages(gfp, PMD_PAGE_ORDER);
+	if (!page)
+		return -ENOMEM;
+
+	addr = (unsigned long)page_address(page);
+	split_page(page, PMD_PAGE_ORDER);
+
+	err = gen_pool_add(pool, addr, PMD_SIZE, NUMA_NO_NODE);
+	if (err) {
+		__free_pages(page, PMD_PAGE_ORDER);
+		return err;
+	}
+
+	__kernel_map_pages(page, nr_pages, 0);
+
+	return 0;
+}
+
+static struct page *secretmem_alloc_page(struct secretmem_ctx *ctx,
+					 gfp_t gfp)
+{
+	struct gen_pool *pool = ctx->pool;
+	unsigned long addr;
+	struct page *page;
+	int err;
+
+	if (gen_pool_avail(pool) < PAGE_SIZE) {
+		err = secretmem_pool_increase(ctx, gfp);
+		if (err)
+			return NULL;
+	}
+
+	addr = gen_pool_alloc(pool, PAGE_SIZE);
+	if (!addr)
+		return NULL;
+
+	page = virt_to_page(addr);
+	get_page(page);
+
+	return page;
 }
 
 static vm_fault_t secretmem_fault(struct vm_fault *vmf)
 {
+	struct secretmem_ctx *ctx = vmf->vma->vm_file->private_data;
 	struct address_space *mapping = vmf->vma->vm_file->f_mapping;
 	struct inode *inode = file_inode(vmf->vma->vm_file);
 	pgoff_t offset = vmf->pgoff;
-	unsigned long addr;
 	struct page *page;
 	int ret = 0;
 

@@ -51,7 +94,7 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
 
 	page = find_get_entry(mapping, offset);
 	if (!page) {
-		page = secretmem_alloc_page(vmf->gfp_mask);
+		page = secretmem_alloc_page(ctx, vmf->gfp_mask);
 		if (!page)
 			return vmf_error(-ENOMEM);
 

@@ -59,14 +102,8 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
 		if (unlikely(ret))
 			goto err_put_page;
 
-		ret = set_direct_map_invalid_noflush(page);
-		if (ret)
-			goto err_del_page_cache;
-
-		addr = (unsigned long)page_address(page);
-		flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
-
 		__SetPageUptodate(page);
+		set_page_private(page, (unsigned long)ctx);
 
 		ret = VM_FAULT_LOCKED;
 	}

@@ -74,8 +111,6 @@ static vm_fault_t secretmem_fault(struct vm_fault *vmf)
 	vmf->page = page;
 	return ret;
 
-err_del_page_cache:
-	delete_from_page_cache(page);
 err_put_page:
 	put_page(page);
 	return vmf_error(ret);

@@ -134,7 +169,11 @@ static int secretmem_migratepage(struct address_space *mapping,
 
 static void secretmem_freepage(struct page *page)
 {
-	set_direct_map_default_noflush(page);
+	unsigned long addr = (unsigned long)page_address(page);
+	struct secretmem_ctx *ctx = (struct secretmem_ctx *)page_private(page);
+	struct gen_pool *pool = ctx->pool;
+
+	gen_pool_free(pool, addr, PAGE_SIZE);
 }
 
 static const struct address_space_operations secretmem_aops = {

@@ -159,13 +198,18 @@ static struct file *secretmem_file_create(unsigned long flags)
 	if (!ctx)
 		goto err_free_inode;
 
+	ctx->pool = gen_pool_create(PAGE_SHIFT, NUMA_NO_NODE);
+	if (!ctx->pool)
+		goto err_free_ctx;
+
 	file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
 				 O_RDWR, &secretmem_fops);
 	if (IS_ERR(file))
-		goto err_free_ctx;
+		goto err_free_pool;
 
 	mapping_set_unevictable(inode->i_mapping);
 
+	inode->i_private = ctx;
 	inode->i_mapping->private_data = ctx;
 	inode->i_mapping->a_ops = &secretmem_aops;
 

@@ -179,6 +223,8 @@ static struct file *secretmem_file_create(unsigned long flags)
 
 	return file;
 
+err_free_pool:
+	gen_pool_destroy(ctx->pool);
 err_free_ctx:
 	kfree(ctx);
 err_free_inode:

@@ -223,11 +269,34 @@ SYSCALL_DEFINE1(memfd_secret, unsigned long, flags)
 	return err;
 }
 
+static void secretmem_cleanup_chunk(struct gen_pool *pool,
+				    struct gen_pool_chunk *chunk, void *data)
+{
+	unsigned long start = chunk->start_addr;
+	unsigned long end = chunk->end_addr;
+	unsigned long nr_pages, addr;
+
+	nr_pages = (end - start + 1) / PAGE_SIZE;
+	__kernel_map_pages(virt_to_page(start), nr_pages, 1);
+
+	for (addr = start; addr < end; addr += PAGE_SIZE)
+		put_page(virt_to_page(addr));
+}
+
+static void secretmem_cleanup_pool(struct secretmem_ctx *ctx)
+{
+	struct gen_pool *pool = ctx->pool;
+
+	gen_pool_for_each_chunk(pool, secretmem_cleanup_chunk, ctx);
+	gen_pool_destroy(pool);
+}
+
 static void secretmem_evict_inode(struct inode *inode)
 {
 	struct secretmem_ctx *ctx = inode->i_private;
 
 	truncate_inode_pages_final(&inode->i_data);
+	secretmem_cleanup_pool(ctx);
 	clear_inode(inode);
 	kfree(ctx);
 }

-- 
2.26.2

From: Mike Rapoport <redacted>

Taking pages out from the direct map and bringing them back may create
undesired fragmentation and usage of the smaller pages in the direct
mapping of the physical memory.

This can be avoided if a significantly large area of the physical memory
would be reserved for secretmem purposes at boot time.

Add ability to reserve physical memory for secretmem at boot time using
"secretmem" kernel parameter and then use that reserved memory as a global
pool for secret memory needs.

Signed-off-by: Mike Rapoport <redacted>
---
 mm/secretmem.c | 134 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 126 insertions(+), 8 deletions(-)

diff --git a/mm/secretmem.c b/mm/secretmem.c
index da609701e10e..35616e3982a4 100644
--- a/mm/secretmem.c
+++ b/mm/secretmem.c

@@ -8,6 +8,7 @@
 #include <linux/pagemap.h>
 #include <linux/genalloc.h>
 #include <linux/syscalls.h>
+#include <linux/memblock.h>
 #include <linux/pseudo_fs.h>
 #include <linux/set_memory.h>
 #include <linux/sched/signal.h>

@@ -30,6 +31,39 @@ struct secretmem_ctx {
 	unsigned int mode;
 };
 
+struct secretmem_pool {
+	struct gen_pool *pool;
+	unsigned long reserved_size;
+	void *reserved;
+};
+
+static struct secretmem_pool secretmem_pool;
+
+static struct page *secretmem_alloc_huge_page(gfp_t gfp)
+{
+	struct gen_pool *pool = secretmem_pool.pool;
+	unsigned long addr = 0;
+	struct page *page = NULL;
+
+	if (pool) {
+		if (gen_pool_avail(pool) < PMD_SIZE)
+			return NULL;
+
+		addr = gen_pool_alloc(pool, PMD_SIZE);
+		if (!addr)
+			return NULL;
+
+		page = virt_to_page(addr);
+	} else {
+		page = alloc_pages(gfp, PMD_PAGE_ORDER);
+
+		if (page)
+			split_page(page, PMD_PAGE_ORDER);
+	}
+
+	return page;
+}
+
 static int secretmem_pool_increase(struct secretmem_ctx *ctx, gfp_t gfp)
 {
 	unsigned long nr_pages = (1 << PMD_PAGE_ORDER);

@@ -38,12 +72,11 @@ static int secretmem_pool_increase(struct secretmem_ctx *ctx, gfp_t gfp)
 	struct page *page;
 	int err;
 
-	page = alloc_pages(gfp, PMD_PAGE_ORDER);
+	page = secretmem_alloc_huge_page(gfp);
 	if (!page)
 		return -ENOMEM;
 
 	addr = (unsigned long)page_address(page);
-	split_page(page, PMD_PAGE_ORDER);
 
 	err = gen_pool_add(pool, addr, PMD_SIZE, NUMA_NO_NODE);
 	if (err) {

@@ -269,11 +302,13 @@ SYSCALL_DEFINE1(memfd_secret, unsigned long, flags)
 	return err;
 }
 
-static void secretmem_cleanup_chunk(struct gen_pool *pool,
-				    struct gen_pool_chunk *chunk, void *data)
+static void secretmem_recycle_range(unsigned long start, unsigned long end)
+{
+	gen_pool_free(secretmem_pool.pool, start, PMD_SIZE);
+}
+
+static void secretmem_release_range(unsigned long start, unsigned long end)
 {
-	unsigned long start = chunk->start_addr;
-	unsigned long end = chunk->end_addr;
 	unsigned long nr_pages, addr;
 
 	nr_pages = (end - start + 1) / PAGE_SIZE;

@@ -283,6 +318,18 @@ static void secretmem_cleanup_chunk(struct gen_pool *pool,
 		put_page(virt_to_page(addr));
 }
 
+static void secretmem_cleanup_chunk(struct gen_pool *pool,
+				    struct gen_pool_chunk *chunk, void *data)
+{
+	unsigned long start = chunk->start_addr;
+	unsigned long end = chunk->end_addr;
+
+	if (secretmem_pool.pool)
+		secretmem_recycle_range(start, end);
+	else
+		secretmem_release_range(start, end);
+}
+
 static void secretmem_cleanup_pool(struct secretmem_ctx *ctx)
 {
 	struct gen_pool *pool = ctx->pool;

@@ -322,14 +369,85 @@ static struct file_system_type secretmem_fs = {
 	.kill_sb	= kill_anon_super,
 };
 
+static int secretmem_reserved_mem_init(void)
+{
+	struct gen_pool *pool;
+	struct page *page;
+	void *addr;
+	int err;
+
+	if (!secretmem_pool.reserved)
+		return 0;
+
+	pool = gen_pool_create(PMD_SHIFT, NUMA_NO_NODE);
+	if (!pool)
+		return -ENOMEM;
+
+	err = gen_pool_add(pool, (unsigned long)secretmem_pool.reserved,
+			   secretmem_pool.reserved_size, NUMA_NO_NODE);
+	if (err)
+		goto err_destroy_pool;
+
+	for (addr = secretmem_pool.reserved;
+	     addr < secretmem_pool.reserved + secretmem_pool.reserved_size;
+	     addr += PAGE_SIZE) {
+		page = virt_to_page(addr);
+		__ClearPageReserved(page);
+		set_page_count(page, 1);
+	}
+
+	secretmem_pool.pool = pool;
+	page = virt_to_page(secretmem_pool.reserved);
+	__kernel_map_pages(page, secretmem_pool.reserved_size / PAGE_SIZE, 0);
+	return 0;
+
+err_destroy_pool:
+	gen_pool_destroy(pool);
+	return err;
+}
+
 static int secretmem_init(void)
 {
-	int ret = 0;
+	int ret;
+
+	ret = secretmem_reserved_mem_init();
+	if (ret)
+		return ret;
 
 	secretmem_mnt = kern_mount(&secretmem_fs);
-	if (IS_ERR(secretmem_mnt))
+	if (IS_ERR(secretmem_mnt)) {
+		gen_pool_destroy(secretmem_pool.pool);
 		ret = PTR_ERR(secretmem_mnt);
+	}
 
 	return ret;
 }
 fs_initcall(secretmem_init);
+
+static int __init secretmem_setup(char *str)
+{
+	phys_addr_t align = PMD_SIZE;
+	unsigned long reserved_size;
+	void *reserved;
+
+	reserved_size = memparse(str, NULL);
+	if (!reserved_size)
+		return 0;
+
+	if (reserved_size * 2 > PUD_SIZE)
+		align = PUD_SIZE;
+
+	reserved = memblock_alloc(reserved_size, align);
+	if (!reserved) {
+		pr_err("failed to reserve %lu bytes\n", secretmem_pool.reserved_size);
+		return 0;
+	}
+
+	secretmem_pool.reserved_size = reserved_size;
+	secretmem_pool.reserved = reserved;
+
+	pr_info("reserved %luM\n", reserved_size >> 20);
+
+	return 1;
+}
+__setup("secretmem=", secretmem_setup);

-- 
2.26.2

From: Mike Rapoport <redacted>

Taking pages out from the direct map and bringing them back may create
undesired fragmentation and usage of the smaller pages in the direct
mapping of the physical memory.

This can be avoided if a significantly large area of the physical memory
would be reserved for secretmem purposes at boot time.

Add ability to reserve physical memory for secretmem at boot time using
"secretmem" kernel parameter and then use that reserved memory as a global
pool for secret memory needs.

Signed-off-by: Mike Rapoport <redacted>
---
 Documentation/admin-guide/kernel-parameters.txt | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index fb95fad81c79..6f3c2f28160f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -4548,6 +4548,10 @@
 			Format: integer between 0 and 10
 			Default is 0.
 
+	secretmem=n[KMG]
+			[KNL,BOOT] Reserve specified amount of memory to
+			back mappings of secret memory.
+
 	skew_tick=	[KNL] Offset the periodic timer tick per cpu to mitigate
 			xtime_lock contention on larger systems, and/or RCU lock
 			contention on all systems with CONFIG_MAXSMP set.

-- 
2.26.2

Oops, something went wrong with the rebase, this should have been
squashed into the previous patch...

On Mon, Jul 27, 2020 at 07:29:35PM +0300, Mike Rapoport wrote:

-- 
Sincerely yours,
Mike.