On Mon, Jul 18, 2016 at 6:06 PM, Laura Abbott [off-list ref] wrote:

On 07/15/2016 02:44 PM, Kees Cook wrote:

quoted

This is the start of porting PAX_USERCOPY into the mainline kernel. This
is the first set of features, controlled by CONFIG_HARDENED_USERCOPY. The
work is based on code by PaX Team and Brad Spengler, and an earlier port
from Casey Schaufler. Additional non-slab page tests are from Rik van
Riel.

This patch contains the logic for validating several conditions when
performing copy_to_user() and copy_from_user() on the kernel object
being copied to/from:
- address range doesn't wrap around
- address range isn't NULL or zero-allocated (with a non-zero copy size)
- if on the slab allocator:
  - object size must be less than or equal to copy size (when check is
    implemented in the allocator, which appear in subsequent patches)
- otherwise, object must not span page allocations
- if on the stack
  - object must not extend before/after the current process task
  - object must be contained by the current stack frame (when there is
    arch/build support for identifying stack frames)
- object must not overlap with kernel text

Signed-off-by: Kees Cook <redacted>
Tested-By: Valdis Kletnieks <redacted>
Tested-by: Michael Ellerman <mpe@ellerman.id.au>
---
 arch/Kconfig                |   7 ++
 include/linux/slab.h        |  12 +++
 include/linux/thread_info.h |  15 +++
 mm/Makefile                 |   4 +
 mm/usercopy.c               | 234
++++++++++++++++++++++++++++++++++++++++++++
 security/Kconfig            |  28 ++++++
 6 files changed, 300 insertions(+)
 create mode 100644 mm/usercopy.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 5e2776562035..195ee4cc939a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig

@@ -433,6 +433,13 @@ config HAVE_ARCH_WITHIN_STACK_FRAMES
          and similar) by implementing an inline

arch_within_stack_frames(),
          which is used by CONFIG_HARDENED_USERCOPY.

+config HAVE_ARCH_LINEAR_KERNEL_MAPPING
+       bool
+       help
+         An architecture should select this if it has a secondary linear
+         mapping of the kernel text. This is used to verify that kernel
+         text exposures are not visible under CONFIG_HARDENED_USERCOPY.
+
 config HAVE_CONTEXT_TRACKING
        bool
        help

diff --git a/include/linux/slab.h b/include/linux/slab.h
index aeb3e6d00a66..96a16a3fb7cb 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h

@@ -155,6 +155,18 @@ void kfree(const void *);
 void kzfree(const void *);
 size_t ksize(const void *);

+#ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
+const char *__check_heap_object(const void *ptr, unsigned long n,
+                               struct page *page);
+#else
+static inline const char *__check_heap_object(const void *ptr,
+                                             unsigned long n,
+                                             struct page *page)
+{
+       return NULL;
+}
+#endif
+
 /*
  * Some archs want to perform DMA into kmalloc caches and need a

guaranteed
  * alignment larger than the alignment of a 64-bit integer.

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 3d5c80b4391d..f24b99eac969 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h

@@ -155,6 +155,21 @@ static inline int arch_within_stack_frames(const void

* const stack,
 }
 #endif

+#ifdef CONFIG_HARDENED_USERCOPY
+extern void __check_object_size(const void *ptr, unsigned long n,
+                                       bool to_user);
+
+static inline void check_object_size(const void *ptr, unsigned long n,
+                                    bool to_user)
+{
+       __check_object_size(ptr, n, to_user);
+}
+#else
+static inline void check_object_size(const void *ptr, unsigned long n,
+                                    bool to_user)
+{ }
+#endif /* CONFIG_HARDENED_USERCOPY */
+
 #endif /* __KERNEL__ */

 #endif /* _LINUX_THREAD_INFO_H */

diff --git a/mm/Makefile b/mm/Makefile
index 78c6f7dedb83..32d37247c7e5 100644
--- a/mm/Makefile
+++ b/mm/Makefile

@@ -21,6 +21,9 @@ KCOV_INSTRUMENT_memcontrol.o := n
 KCOV_INSTRUMENT_mmzone.o := n
 KCOV_INSTRUMENT_vmstat.o := n

+# Since __builtin_frame_address does work as used, disable the warning.
+CFLAGS_usercopy.o += $(call cc-disable-warning, frame-address)
+
 mmu-y                  := nommu.o
 mmu-$(CONFIG_MMU)      := gup.o highmem.o memory.o mincore.o \
                           mlock.o mmap.o mprotect.o mremap.o msync.o

rmap.o \

@@ -99,3 +102,4 @@ obj-$(CONFIG_USERFAULTFD) += userfaultfd.o
 obj-$(CONFIG_IDLE_PAGE_TRACKING) += page_idle.o
 obj-$(CONFIG_FRAME_VECTOR) += frame_vector.o
 obj-$(CONFIG_DEBUG_PAGE_REF) += debug_page_ref.o
+obj-$(CONFIG_HARDENED_USERCOPY) += usercopy.o

diff --git a/mm/usercopy.c b/mm/usercopy.c
new file mode 100644
index 000000000000..e4bf4e7ccdf6
--- /dev/null
+++ b/mm/usercopy.c

@@ -0,0 +1,234 @@
+/*
+ * This implements the various checks for CONFIG_HARDENED_USERCOPY*,
+ * which are designed to protect kernel memory from needless exposure
+ * and overwrite under many unintended conditions. This code is based
+ * on PAX_USERCOPY, which is:
+ *
+ * Copyright (C) 2001-2016 PaX Team, Bradley Spengler, Open Source
+ * Security Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <asm/sections.h>
+
+enum {
+       BAD_STACK = -1,
+       NOT_STACK = 0,
+       GOOD_FRAME,
+       GOOD_STACK,
+};
+
+/*
+ * Checks if a given pointer and length is contained by the current
+ * stack frame (if possible).
+ *
+ *     0: not at all on the stack
+ *     1: fully within a valid stack frame
+ *     2: fully on the stack (when can't do frame-checking)
+ *     -1: error condition (invalid stack position or bad stack frame)
+ */

Nit: update comments to match enum (BAD_STACK instead of -1 etc.)

Ah, yes, thanks. I will fix this.

quoted

+static noinline int check_stack_object(const void *obj, unsigned long
len)
+{
+       const void * const stack = task_stack_page(current);
+       const void * const stackend = stack + THREAD_SIZE;
+       int ret;
+
+       /* Object is not on the stack at all. */
+       if (obj + len <= stack || stackend <= obj)
+               return NOT_STACK;
+
+       /*
+        * Reject: object partially overlaps the stack (passing the
+        * the check above means at least one end is within the stack,
+        * so if this check fails, the other end is outside the stack).
+        */
+       if (obj < stack || stackend < obj + len)
+               return BAD_STACK;
+
+       /* Check if object is safely within a valid frame. */
+       ret = arch_within_stack_frames(stack, stackend, obj, len);
+       if (ret)
+               return ret;
+
+       return GOOD_STACK;
+}
+
+static void report_usercopy(const void *ptr, unsigned long len,
+                           bool to_user, const char *type)
+{
+       pr_emerg("kernel memory %s attempt detected %s %p (%s) (%lu
bytes)\n",
+               to_user ? "exposure" : "overwrite",
+               to_user ? "from" : "to", ptr, type ? : "unknown", len);
+       /*
+        * For greater effect, it would be nice to do do_group_exit(),
+        * but BUG() actually hooks all the lock-breaking and per-arch
+        * Oops code, so that is used here instead.
+        */
+       BUG();
+}
+
+/* Returns true if any portion of [ptr,ptr+n) over laps with [low,high).
*/
+static bool overlaps(const void *ptr, unsigned long n, unsigned long low,
+                    unsigned long high)
+{
+       unsigned long check_low = (uintptr_t)ptr;
+       unsigned long check_high = check_low + n;
+
+       /* Does not overlap if entirely above or entirely below. */
+       if (check_low >= high || check_high < low)
+               return false;
+
+       return true;
+}
+
+/* Is this address range in the kernel text area? */
+static inline const char *check_kernel_text_object(const void *ptr,
+                                                  unsigned long n)
+{
+       unsigned long textlow = (unsigned long)_stext;
+       unsigned long texthigh = (unsigned long)_etext;
+
+       if (overlaps(ptr, n, textlow, texthigh))
+               return "<kernel text>";
+
+#ifdef HAVE_ARCH_LINEAR_KERNEL_MAPPING
+       /* Check against linear mapping as well. */
+       if (overlaps(ptr, n, (unsigned long)__va(__pa(textlow)),
+                    (unsigned long)__va(__pa(texthigh))))
+               return "<linear kernel text>";
+#endif
+
+       return NULL;
+}
+
+static inline const char *check_bogus_address(const void *ptr, unsigned
long n)
+{
+       /* Reject if object wraps past end of memory. */
+       if (ptr + n < ptr)
+               return "<wrapped address>";
+
+       /* Reject if NULL or ZERO-allocation. */
+       if (ZERO_OR_NULL_PTR(ptr))
+               return "<null>";
+
+       return NULL;
+}
+
+static inline const char *check_heap_object(const void *ptr, unsigned
long n,
+                                           bool to_user)
+{
+       struct page *page, *endpage;
+       const void *end = ptr + n - 1;
+
+       if (!virt_addr_valid(ptr))
+               return NULL;
+
+       page = virt_to_head_page(ptr);
+
+       /* Check slab allocator for flags and size. */
+       if (PageSlab(page))
+               return __check_heap_object(ptr, n, page);
+
+       /*
+        * Sometimes the kernel data regions are not marked Reserved (see
+        * check below). And sometimes [_sdata,_edata) does not cover
+        * rodata and/or bss, so check each range explicitly.
+        */
+
+       /* Allow reads of kernel rodata region (if not marked as
Reserved). */
+       if (ptr >= (const void *)__start_rodata &&
+           end <= (const void *)__end_rodata) {
+               if (!to_user)
+                       return "<rodata>";
+               return NULL;
+       }
+
+       /* Allow kernel data region (if not marked as Reserved). */
+       if (ptr >= (const void *)_sdata && end <= (const void *)_edata)
+               return NULL;
+
+       /* Allow kernel bss region (if not marked as Reserved). */
+       if (ptr >= (const void *)__bss_start &&
+           end <= (const void *)__bss_stop)
+               return NULL;
+
+       /* Is the object wholly within one base page? */
+       if (likely(((unsigned long)ptr & (unsigned long)PAGE_MASK) ==
+                  ((unsigned long)end & (unsigned long)PAGE_MASK)))
+               return NULL;
+
+       /* Allow if start and end are inside the same compound page. */
+       endpage = virt_to_head_page(end);
+       if (likely(endpage == page))
+               return NULL;
+
+       /*
+        * Reject if range is not Reserved (i.e. special or device
memory),
+        * since then the object spans several independently allocated
pages.
+        */
+       for (; ptr <= end ; ptr += PAGE_SIZE, page =
virt_to_head_page(ptr)) {
+               if (!PageReserved(page))
+                       return "<spans multiple pages>";
+       }
+

This doesn't work when copying CMA allocated memory since CMA purposely
allocates larger than a page block size without setting head pages.
Given CMA may be used with drivers doing zero copy buffers, I think it
should be permitted.

Something like the following lets it pass (I can clean up and submit
the is_migrate_cma_page APIs as a separate patch for review)

Yeah, this would be great. I'd rather use an accessor to check this
than a direct check for MIGRATE_CMA.

Yeah, I'll modify this a bit so that which type it starts as is
maintained for all pages (rather than allowing to flip back and forth
-- even though that is likely impossible).

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security