[PATCH v5 0/5] Implement STRICT_MODULE_RWX for powerpc

v4 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198268.html
v3 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198023.html

Changes since v4:
	[1/5]: Addressed review comments from Michael Ellerman (thanks!)
	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
	       STRICT_MODULE_RWX being *on by default* in cases where
	       STRICT_KERNEL_RWX is *unavailable*
	[5/5]: split skiroot_defconfig changes out into its own patch

The whole Kconfig situation is really weird and confusing, I believe the
correct resolution is to change arch/Kconfig but the consequences are so
minor that I don't think it's worth it, especially given that I expect
powerpc to have mandatory strict RWX Soon(tm).

Russell Currey (5):
  powerpc/mm: Implement set_memory() routines
  powerpc/kprobes: Mark newly allocated probes as RO
  powerpc/mm/ptdump: debugfs handler for W+X checks at runtime
  powerpc: Set ARCH_HAS_STRICT_MODULE_RWX
  powerpc/configs: Enable STRICT_MODULE_RWX in skiroot_defconfig

 arch/powerpc/Kconfig                   |  2 +
 arch/powerpc/Kconfig.debug             |  6 +-
 arch/powerpc/configs/skiroot_defconfig |  1 +
 arch/powerpc/include/asm/set_memory.h  | 32 +++++++++++
 arch/powerpc/kernel/kprobes.c          |  3 +
 arch/powerpc/mm/Makefile               |  1 +
 arch/powerpc/mm/pageattr.c             | 77 ++++++++++++++++++++++++++
 arch/powerpc/mm/ptdump/ptdump.c        | 21 ++++++-
 8 files changed, 140 insertions(+), 3 deletions(-)
 create mode 100644 arch/powerpc/include/asm/set_memory.h
 create mode 100644 arch/powerpc/mm/pageattr.c

-- 
2.23.0

With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
W+X page at boot by default.  This can be tested with
CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
kernel log during boot.

powerpc doesn't implement its own alloc() for kprobes like other
architectures do, but we couldn't immediately mark RO anyway since we do
a memcpy to the page we allocate later.  After that, nothing should be
allowed to modify the page, and write permissions are removed well
before the kprobe is armed.

Thus mark newly allocated probes as read-only once it's safe to do so.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/kernel/kprobes.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 2d27ec4feee4..2610496de7c7 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c

@@ -24,6 +24,7 @@
 #include <asm/sstep.h>
 #include <asm/sections.h>
 #include <linux/uaccess.h>
+#include <linux/set_memory.h>
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

@@ -131,6 +132,8 @@ int arch_prepare_kprobe(struct kprobe *p)
 			(unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t));
 	}
 
+	set_memory_ro((unsigned long)p->ainsn.insn, 1);
+
 	p->ainsn.boostable = 0;
 	return ret;
 }

-- 
2.23.0

Russell Currey [off-list ref] writes:

With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
W+X page at boot by default.  This can be tested with
CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
kernel log during boot.

powerpc doesn't implement its own alloc() for kprobes like other
architectures do, but we couldn't immediately mark RO anyway since we do
a memcpy to the page we allocate later.  After that, nothing should be
allowed to modify the page, and write permissions are removed well
before the kprobe is armed.

Thus mark newly allocated probes as read-only once it's safe to do so.

So if I've got the flow right here:

register[_aggr]_kprobe
 -> prepare_kprobe
  -> arch_prepare_kprobe
       perform memcpy
       mark as readonly, after which no-one touches writes to the memory

which all seems right to me.

I have been trying to check if optprobes need special handling: it looks
like the buffer for them lives in kernel text, not dynamically allocated
memory, so it should be protected by the usual Strict RWX protections
without special treatment here.

So lgtm.

Reviewed-by: Daniel Axtens <redacted>

Regards,
Daniel

Russell Currey [off-list ref] writes:

That comes from:
	p->ainsn.insn = get_insn_slot();


Which ends up in __get_insn_slot() I think. And that looks very much
like it's going to hand out multiple slots per page, which isn't going
to work because you've just marked the whole page RO.

So I would expect this to crash on the 2nd kprobe that's installed. Have
you tested it somehow?

I think this code should just use patch_instruction() rather than
memcpy().

cheers

Michael Ellerman [off-list ref] writes:

Russell Currey [off-list ref] writes:

quoted

With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
W+X page at boot by default.  This can be tested with
CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
kernel log during boot.

powerpc doesn't implement its own alloc() for kprobes like other
architectures do, but we couldn't immediately mark RO anyway since we do
a memcpy to the page we allocate later.  After that, nothing should be
allowed to modify the page, and write permissions are removed well
before the kprobe is armed.

Thus mark newly allocated probes as read-only once it's safe to do so.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/kernel/kprobes.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 2d27ec4feee4..2610496de7c7 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c

@@ -24,6 +24,7 @@
 #include <asm/sstep.h>
 #include <asm/sections.h>
 #include <linux/uaccess.h>
+#include <linux/set_memory.h>
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

@@ -131,6 +132,8 @@ int arch_prepare_kprobe(struct kprobe *p)
 			(unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t));
 	}
 
+	set_memory_ro((unsigned long)p->ainsn.insn, 1);
+

That comes from:
	p->ainsn.insn = get_insn_slot();


Which ends up in __get_insn_slot() I think. And that looks very much
like it's going to hand out multiple slots per page, which isn't going
to work because you've just marked the whole page RO.

So I would expect this to crash on the 2nd kprobe that's installed. Have
you tested it somehow?

I'm not sure if this is the issue I was talking about, but it doesn't
survive ftracetest:

  [ 1139.576047] ------------[ cut here ]------------
  [ 1139.576322] kernel BUG at mm/memory.c:2036!
  cpu 0x1f: Vector: 700 (Program Check) at [c000001fd6c675d0]
      pc: c00000000035d018: apply_to_page_range+0x318/0x610
      lr: c0000000000900bc: change_memory_attr+0x4c/0x70
      sp: c000001fd6c67860
     msr: 9000000000029033
    current = 0xc000001fa4a47880
    paca    = 0xc000001ffffe5c80   irqmask: 0x03   irq_happened: 0x01
      pid   = 7168, comm = ftracetest
  kernel BUG at mm/memory.c:2036!
  Linux version 5.4.0-gcc-8.2.0-11694-gf1f9aa266811 (michael@Raptor-2.ozlabs.ibm.com) (gcc version 8.2.0 (crosstool-NG 1.24.0-rc1.16-9627a04)) #1384 SMP Thu Dec 5 22:11:09 AEDT 2019
  enter ? for help
  [c000001fd6c67940] c0000000000900bc change_memory_attr+0x4c/0x70
  [c000001fd6c67970] c000000000053c48 arch_prepare_kprobe+0xb8/0x120
  [c000001fd6c679e0] c00000000022f718 register_kprobe+0x608/0x790
  [c000001fd6c67a40] c00000000022fc50 register_kretprobe+0x230/0x350
  [c000001fd6c67a80] c0000000002849b4 __register_trace_kprobe+0xf4/0x1a0
  [c000001fd6c67af0] c000000000285b18 trace_kprobe_create+0x738/0xf70
  [c000001fd6c67c30] c000000000286378 create_or_delete_trace_kprobe+0x28/0x70
  [c000001fd6c67c50] c00000000025f024 trace_run_command+0xc4/0xe0
  [c000001fd6c67ca0] c00000000025f128 trace_parse_run_command+0xe8/0x230
  [c000001fd6c67d40] c0000000002845d0 probes_write+0x20/0x40
  [c000001fd6c67d60] c0000000003eef4c __vfs_write+0x3c/0x70
  [c000001fd6c67d80] c0000000003f26a0 vfs_write+0xd0/0x200
  [c000001fd6c67dd0] c0000000003f2a3c ksys_write+0x7c/0x140
  [c000001fd6c67e20] c00000000000b9e0 system_call+0x5c/0x68
  --- Exception: c01 (System Call) at 00007fff8f06e420
  SP (7ffff93d6830) is in userspace
  1f:mon> client_loop: send disconnect: Broken pipe


Sorry I didn't get any more info on the crash, I lost the console and
then some CI bot stole the machine 8)

You should be able to reproduce just by running ftracetest.

cheers

On Fri, 2019-12-06 at 10:47 +1100, Michael Ellerman wrote:

Michael Ellerman [off-list ref] writes:

quoted

Russell Currey [off-list ref] writes:

quoted

With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will
be one
W+X page at boot by default.  This can be tested with
CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking
the
kernel log during boot.

powerpc doesn't implement its own alloc() for kprobes like other
architectures do, but we couldn't immediately mark RO anyway
since we do
a memcpy to the page we allocate later.  After that, nothing
should be
allowed to modify the page, and write permissions are removed
well
before the kprobe is armed.

Thus mark newly allocated probes as read-only once it's safe to
do so.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/kernel/kprobes.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes.c

b/arch/powerpc/kernel/kprobes.c
index 2d27ec4feee4..2610496de7c7 100644

--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c

@@ -24,6 +24,7 @@
 #include <asm/sstep.h>
 #include <asm/sections.h>
 #include <linux/uaccess.h>
+#include <linux/set_memory.h>
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);

@@ -131,6 +132,8 @@ int arch_prepare_kprobe(struct kprobe *p)
 			(unsigned long)p->ainsn.insn +

sizeof(kprobe_opcode_t));
 	}
 
+	set_memory_ro((unsigned long)p->ainsn.insn, 1);
+

That comes from:
	p->ainsn.insn = get_insn_slot();


Which ends up in __get_insn_slot() I think. And that looks very
much
like it's going to hand out multiple slots per page, which isn't
going
to work because you've just marked the whole page RO.

So I would expect this to crash on the 2nd kprobe that's installed.
Have
you tested it somehow?

I'm not sure if this is the issue I was talking about, but it doesn't
survive ftracetest:

  [ 1139.576047] ------------[ cut here ]------------
  [ 1139.576322] kernel BUG at mm/memory.c:2036!
  cpu 0x1f: Vector: 700 (Program Check) at [c000001fd6c675d0]
      pc: c00000000035d018: apply_to_page_range+0x318/0x610
      lr: c0000000000900bc: change_memory_attr+0x4c/0x70
      sp: c000001fd6c67860
     msr: 9000000000029033
    current = 0xc000001fa4a47880
    paca    = 0xc000001ffffe5c80   irqmask: 0x03   irq_happened: 0x01
      pid   = 7168, comm = ftracetest
  kernel BUG at mm/memory.c:2036!
  Linux version 5.4.0-gcc-8.2.0-11694-gf1f9aa266811 (
michael@Raptor-2.ozlabs.ibm.com) (gcc version 8.2.0 (crosstool-NG
1.24.0-rc1.16-9627a04)) #1384 SMP Thu Dec 5 22:11:09 AEDT 2019
  enter ? for help
  [c000001fd6c67940] c0000000000900bc change_memory_attr+0x4c/0x70
  [c000001fd6c67970] c000000000053c48 arch_prepare_kprobe+0xb8/0x120
  [c000001fd6c679e0] c00000000022f718 register_kprobe+0x608/0x790
  [c000001fd6c67a40] c00000000022fc50 register_kretprobe+0x230/0x350
  [c000001fd6c67a80] c0000000002849b4
__register_trace_kprobe+0xf4/0x1a0
  [c000001fd6c67af0] c000000000285b18 trace_kprobe_create+0x738/0xf70
  [c000001fd6c67c30] c000000000286378
create_or_delete_trace_kprobe+0x28/0x70
  [c000001fd6c67c50] c00000000025f024 trace_run_command+0xc4/0xe0
  [c000001fd6c67ca0] c00000000025f128
trace_parse_run_command+0xe8/0x230
  [c000001fd6c67d40] c0000000002845d0 probes_write+0x20/0x40
  [c000001fd6c67d60] c0000000003eef4c __vfs_write+0x3c/0x70
  [c000001fd6c67d80] c0000000003f26a0 vfs_write+0xd0/0x200
  [c000001fd6c67dd0] c0000000003f2a3c ksys_write+0x7c/0x140
  [c000001fd6c67e20] c00000000000b9e0 system_call+0x5c/0x68
  --- Exception: c01 (System Call) at 00007fff8f06e420
  SP (7ffff93d6830) is in userspace
  1f:mon> client_loop: send disconnect: Broken pipe


Sorry I didn't get any more info on the crash, I lost the console and
then some CI bot stole the machine 8)

You should be able to reproduce just by running ftracetest.

The test that blew it up was test.d/kprobe/probepoint.tc for the
record.  It goes away when replacing the memcpy with a
patch_instruction().

cheers

The set_memory_{ro/rw/nx/x}() functions are required for STRICT_MODULE_RWX,
and are generally useful primitives to have.  This implementation is
designed to be completely generic across powerpc's many MMUs.

It's possible that this could be optimised to be faster for specific
MMUs, but the focus is on having a generic and safe implementation for
now.

This implementation does not handle cases where the caller is attempting
to change the mapping of the page it is executing from, or if another
CPU is concurrently using the page being altered.  These cases likely
shouldn't happen, but a more complex implementation with MMU-specific code
could safely handle them, so that is left as a TODO for now.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/Kconfig                  |  1 +
 arch/powerpc/include/asm/set_memory.h | 32 +++++++++++
 arch/powerpc/mm/Makefile              |  1 +
 arch/powerpc/mm/pageattr.c            | 77 +++++++++++++++++++++++++++
 4 files changed, 111 insertions(+)
 create mode 100644 arch/powerpc/include/asm/set_memory.h
 create mode 100644 arch/powerpc/mm/pageattr.c

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 3e56c9c2f16e..8f7005f0d097 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -133,6 +133,7 @@ config PPC
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_MEMBARRIER_CALLBACKS
 	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE && PPC_BOOK3S_64
+	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UACCESS_FLUSHCACHE

diff --git a/arch/powerpc/include/asm/set_memory.h b/arch/powerpc/include/asm/set_memory.h
new file mode 100644
index 000000000000..5230ddb2fefd
--- /dev/null
+++ b/arch/powerpc/include/asm/set_memory.h

@@ -0,0 +1,32 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_POWERPC_SET_MEMORY_H
+#define _ASM_POWERPC_SET_MEMORY_H
+
+#define SET_MEMORY_RO	1
+#define SET_MEMORY_RW	2
+#define SET_MEMORY_NX	3
+#define SET_MEMORY_X	4
+
+int change_memory_attr(unsigned long addr, int numpages, int action);
+
+static inline int set_memory_ro(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_RO);
+}
+
+static inline int set_memory_rw(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_RW);
+}
+
+static inline int set_memory_nx(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_NX);
+}
+
+static inline int set_memory_x(unsigned long addr, int numpages)
+{
+	return change_memory_attr(addr, numpages, SET_MEMORY_X);
+}
+
+#endif

diff --git a/arch/powerpc/mm/Makefile b/arch/powerpc/mm/Makefile
index 5e147986400d..d0a0bcbc9289 100644
--- a/arch/powerpc/mm/Makefile
+++ b/arch/powerpc/mm/Makefile

@@ -20,3 +20,4 @@ obj-$(CONFIG_HIGHMEM)		+= highmem.o
 obj-$(CONFIG_PPC_COPRO_BASE)	+= copro_fault.o
 obj-$(CONFIG_PPC_PTDUMP)	+= ptdump/
 obj-$(CONFIG_KASAN)		+= kasan/
+obj-$(CONFIG_ARCH_HAS_SET_MEMORY) += pageattr.o

diff --git a/arch/powerpc/mm/pageattr.c b/arch/powerpc/mm/pageattr.c
new file mode 100644
index 000000000000..aedd79173a44
--- /dev/null
+++ b/arch/powerpc/mm/pageattr.c

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * MMU-generic set_memory implementation for powerpc
+ *
+ * Copyright 2019, IBM Corporation.
+ */
+
+#include <linux/mm.h>
+#include <linux/set_memory.h>
+
+#include <asm/mmu.h>
+#include <asm/page.h>
+#include <asm/pgtable.h>
+
+
+/*
+ * Updates the attributes of a page in three steps:
+ *
+ * 1. invalidate the page table entry
+ * 2. flush the TLB
+ * 3. install the new entry with the updated attributes
+ *
+ * This is unsafe if the caller is attempting to change the mapping of the
+ * page it is executing from, or if another CPU is concurrently using the
+ * page being altered.
+ *
+ * TODO make the implementation resistant to this.
+ */
+static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
+{
+	int action = *((int *)data);
+	pte_t pte_val;
+	int ret = 0;
+
+	spin_lock(&init_mm.page_table_lock);
+
+	// invalidate the PTE so it's safe to modify
+	pte_val = ptep_get_and_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	// modify the PTE bits as desired, then apply
+	switch (action) {
+	case SET_MEMORY_RO:
+		pte_val = pte_wrprotect(pte_val);
+		break;
+	case SET_MEMORY_RW:
+		pte_val = pte_mkwrite(pte_val);
+		break;
+	case SET_MEMORY_NX:
+		pte_val = pte_exprotect(pte_val);
+		break;
+	case SET_MEMORY_X:
+		pte_val = pte_mkexec(pte_val);
+		break;
+	default:
+		WARN_ON(true);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	set_pte_at(&init_mm, addr, ptep, pte_val);
+out:
+	spin_unlock(&init_mm.page_table_lock);
+	return ret;
+}
+
+int change_memory_attr(unsigned long addr, int numpages, int action)
+{
+	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
+	unsigned long size = numpages * PAGE_SIZE;
+
+	if (!numpages)
+		return 0;
+
+	return apply_to_page_range(&init_mm, start, size, change_page_attr, &action);
+}

-- 
2.23.0

Le 30/10/2019 à 08:31, Russell Currey a écrit :

I don't like too much the way you are making this function more complex 
and less readable with local var, goto, etc ...

You could just keep the v4 version of change_page_attr(), rename it 
__change_page_attr(), then add:

static int change_page_attr(pte_t *ptep, unsigned long addr, void *data)
{
	int ret;

	spin_lock(&init_mm.page_table_lock);
	ret = __change_page_attr(ptep, addr, data);
	spin_unlock(&init_mm.page_table_lock);
	return ret;
}

Christophe

+{
+	int action = *((int *)data);
+	pte_t pte_val;
+	int ret = 0;
+
+	spin_lock(&init_mm.page_table_lock);
+
+	// invalidate the PTE so it's safe to modify
+	pte_val = ptep_get_and_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	// modify the PTE bits as desired, then apply
+	switch (action) {
+	case SET_MEMORY_RO:
+		pte_val = pte_wrprotect(pte_val);
+		break;
+	case SET_MEMORY_RW:
+		pte_val = pte_mkwrite(pte_val);
+		break;
+	case SET_MEMORY_NX:
+		pte_val = pte_exprotect(pte_val);
+		break;
+	case SET_MEMORY_X:
+		pte_val = pte_mkexec(pte_val);
+		break;
+	default:
+		WARN_ON(true);
+		ret = -EINVAL;
+		goto out;
+	}
+
+	set_pte_at(&init_mm, addr, ptep, pte_val);
+out:
+	spin_unlock(&init_mm.page_table_lock);
+	return ret;
+}
+
+int change_memory_attr(unsigned long addr, int numpages, int action)
+{
+	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
+	unsigned long size = numpages * PAGE_SIZE;
+
+	if (!numpages)
+		return 0;
+
+	return apply_to_page_range(&init_mm, start, size, change_page_attr, &action);
+}

Very rudimentary, just

	echo 1 > [debugfs]/check_wx_pages

and check the kernel log.  Useful for testing strict module RWX.

Updated the Kconfig entry to reflect this.

Also fixed a typo.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/Kconfig.debug      |  6 ++++--
 arch/powerpc/mm/ptdump/ptdump.c | 21 ++++++++++++++++++++-
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
index c59920920ddc..dcfe83d4c211 100644
--- a/arch/powerpc/Kconfig.debug
+++ b/arch/powerpc/Kconfig.debug

@@ -370,7 +370,7 @@ config PPC_PTDUMP
 	  If you are unsure, say N.
 
 config PPC_DEBUG_WX
-	bool "Warn on W+X mappings at boot"
+	bool "Warn on W+X mappings at boot & enable manual checks at runtime"
 	depends on PPC_PTDUMP
 	help
 	  Generate a warning if any W+X mappings are found at boot.

@@ -384,7 +384,9 @@ config PPC_DEBUG_WX
 	  of other unfixed kernel bugs easier.
 
 	  There is no runtime or memory usage effect of this option
-	  once the kernel has booted up - it's a one time check.
+	  once the kernel has booted up, it only automatically checks once.
+
+	  Enables the "check_wx_pages" debugfs entry for checking at runtime.
 
 	  If in doubt, say "Y".
 

diff --git a/arch/powerpc/mm/ptdump/ptdump.c b/arch/powerpc/mm/ptdump/ptdump.c
index 2f9ddc29c535..b6cba29ae4a0 100644
--- a/arch/powerpc/mm/ptdump/ptdump.c
+++ b/arch/powerpc/mm/ptdump/ptdump.c

@@ -4,7 +4,7 @@
  *
  * This traverses the kernel pagetables and dumps the
  * information about the used sections of memory to
- * /sys/kernel/debug/kernel_pagetables.
+ * /sys/kernel/debug/kernel_page_tables.
  *
  * Derived from the arm64 implementation:
  * Copyright (c) 2014, The Linux Foundation, Laura Abbott.

@@ -409,6 +409,25 @@ void ptdump_check_wx(void)
 	else
 		pr_info("Checked W+X mappings: passed, no W+X pages found\n");
 }
+
+static int check_wx_debugfs_set(void *data, u64 val)
+{
+	if (val != 1ULL)
+		return -EINVAL;
+
+	ptdump_check_wx();
+
+	return 0;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(check_wx_fops, NULL, check_wx_debugfs_set, "%llu\n");
+
+static int ptdump_check_wx_init(void)
+{
+	return debugfs_create_file("check_wx_pages", 0200, NULL,
+				   NULL, &check_wx_fops) ? 0 : -ENOMEM;
+}
+device_initcall(ptdump_check_wx_init);
 #endif
 
 static int ptdump_init(void)

-- 
2.23.0

skiroot_defconfig is the only powerpc defconfig with STRICT_KERNEL_RWX
enabled, and if you want memory protection for kernel text you'd want it
for modules too, so enable STRICT_MODULE_RWX there.

Signed-off-by: Russell Currey <redacted>
---
 arch/powerpc/configs/skiroot_defconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/configs/skiroot_defconfig b/arch/powerpc/configs/skiroot_defconfig
index 1253482a67c0..719d899081b3 100644
--- a/arch/powerpc/configs/skiroot_defconfig
+++ b/arch/powerpc/configs/skiroot_defconfig

@@ -31,6 +31,7 @@ CONFIG_PERF_EVENTS=y
 CONFIG_SLAB_FREELIST_HARDENED=y
 CONFIG_JUMP_LABEL=y
 CONFIG_STRICT_KERNEL_RWX=y
+CONFIG_STRICT_MODULE_RWX=y
 CONFIG_MODULES=y
 CONFIG_MODULE_UNLOAD=y
 CONFIG_MODULE_SIG=y

-- 
2.23.0

On Wed, 30 Oct 2019 at 07:31, Russell Currey [off-list ref] wrote:

skiroot_defconfig is the only powerpc defconfig with STRICT_KERNEL_RWX
enabled, and if you want memory protection for kernel text you'd want it
for modules too, so enable STRICT_MODULE_RWX there.

Signed-off-by: Russell Currey <redacted>

Acked-by: Joel Stanley <joel@jms.id.au>

To enable strict module RWX on powerpc, set:

    CONFIG_STRICT_MODULE_RWX=y

You should also have CONFIG_STRICT_KERNEL_RWX=y set to have any real
security benefit.

ARCH_HAS_STRICT_MODULE_RWX is set to require ARCH_HAS_STRICT_KERNEL_RWX.
This is due to a quirk in arch/Kconfig and arch/powerpc/Kconfig that
makes STRICT_MODULE_RWX *on by default* in configurations where
STRICT_KERNEL_RWX is *unavailable*.

Since this doesn't make much sense, and module RWX without kernel RWX
doesn't make much sense, having the same dependencies as kernel RWX
works around this problem.

Signed-off-by: Russell Currey <redacted>
---
This means that Daniel's test on book3e64 is pretty useless since we've
gone from being unable to turn it *off* to being unable to turn it *on*.

I think this is the right course of action for now.

 arch/powerpc/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 8f7005f0d097..52028e27f2d3 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -135,6 +135,7 @@ config PPC
 	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE && PPC_BOOK3S_64
 	select ARCH_HAS_SET_MEMORY
 	select ARCH_HAS_STRICT_KERNEL_RWX	if ((PPC_BOOK3S_64 || PPC32) && !RELOCATABLE && !HIBERNATION)
+	select ARCH_HAS_STRICT_MODULE_RWX	if ARCH_HAS_STRICT_KERNEL_RWX
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UACCESS_FLUSHCACHE
 	select ARCH_HAS_UACCESS_MCSAFE		if PPC64

-- 
2.23.0

Le 30/10/2019 à 08:31, Russell Currey a écrit :

v4 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198268.html
v3 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198023.html

Changes since v4:
	[1/5]: Addressed review comments from Michael Ellerman (thanks!)
	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
	       STRICT_MODULE_RWX being *on by default* in cases where
	       STRICT_KERNEL_RWX is *unavailable*
	[5/5]: split skiroot_defconfig changes out into its own patch

The whole Kconfig situation is really weird and confusing, I believe the
correct resolution is to change arch/Kconfig but the consequences are so
minor that I don't think it's worth it, especially given that I expect
powerpc to have mandatory strict RWX Soon(tm).

I'm not such strict RWX can be made mandatory due to the impact it has 
on some subarches:
- On the 8xx, unless all areas are 8Mbytes aligned, there is a 
significant overhead on TLB misses. And Aligning everthing to 8M is a 
waste of RAM which is not acceptable on systems having very few RAM.
- On hash book3s32, we are able to map the kernel BATs. With a few 
alignment constraints, we are able to provide STRICT_KERNEL_RWX. But we 
are unable to provide exec protection on page granularity. Only on 
256Mbytes segments. So for modules, we have to have the vmspace X. It is 
also not possible to have a kernel area RO. Only user areas can be made RO.

Christophe

Russell Currey (5):
   powerpc/mm: Implement set_memory() routines
   powerpc/kprobes: Mark newly allocated probes as RO
   powerpc/mm/ptdump: debugfs handler for W+X checks at runtime
   powerpc: Set ARCH_HAS_STRICT_MODULE_RWX
   powerpc/configs: Enable STRICT_MODULE_RWX in skiroot_defconfig

  arch/powerpc/Kconfig                   |  2 +
  arch/powerpc/Kconfig.debug             |  6 +-
  arch/powerpc/configs/skiroot_defconfig |  1 +
  arch/powerpc/include/asm/set_memory.h  | 32 +++++++++++
  arch/powerpc/kernel/kprobes.c          |  3 +
  arch/powerpc/mm/Makefile               |  1 +
  arch/powerpc/mm/pageattr.c             | 77 ++++++++++++++++++++++++++
  arch/powerpc/mm/ptdump/ptdump.c        | 21 ++++++-
  8 files changed, 140 insertions(+), 3 deletions(-)
  create mode 100644 arch/powerpc/include/asm/set_memory.h
  create mode 100644 arch/powerpc/mm/pageattr.c

On Wed, Oct 30, 2019 at 09:58:19AM +0100, Christophe Leroy wrote:

Le 30/10/2019 à 08:31, Russell Currey a écrit :

quoted

v4 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198268.html
v3 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198023.html

Changes since v4:
	[1/5]: Addressed review comments from Michael Ellerman (thanks!)
	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
	       STRICT_MODULE_RWX being *on by default* in cases where
	       STRICT_KERNEL_RWX is *unavailable*
	[5/5]: split skiroot_defconfig changes out into its own patch

The whole Kconfig situation is really weird and confusing, I believe the
correct resolution is to change arch/Kconfig but the consequences are so
minor that I don't think it's worth it, especially given that I expect
powerpc to have mandatory strict RWX Soon(tm).

I'm not such strict RWX can be made mandatory due to the impact it has on
some subarches:
- On the 8xx, unless all areas are 8Mbytes aligned, there is a significant
overhead on TLB misses. And Aligning everthing to 8M is a waste of RAM which
is not acceptable on systems having very few RAM.
- On hash book3s32, we are able to map the kernel BATs. With a few alignment
constraints, we are able to provide STRICT_KERNEL_RWX. But we are unable to
provide exec protection on page granularity. Only on 256Mbytes segments. So
for modules, we have to have the vmspace X. It is also not possible to have
a kernel area RO. Only user areas can be made RO.

As I understand it, the idea was for it to be mandatory (or at least
default-on) only for the subarches where it wasn't totally insane to
accomplish. :) (I'm not familiar with all the details on the subarchs,
but it sounded like the more modern systems would be the targets for
this?)

-- 
Kees Cook

Le 30/10/2019 à 19:30, Kees Cook a écrit :

On Wed, Oct 30, 2019 at 09:58:19AM +0100, Christophe Leroy wrote:

quoted

Le 30/10/2019 à 08:31, Russell Currey a écrit :

quoted

v4 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198268.html
v3 cover letter: https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198023.html

Changes since v4:
	[1/5]: Addressed review comments from Michael Ellerman (thanks!)
	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
	       STRICT_MODULE_RWX being *on by default* in cases where
	       STRICT_KERNEL_RWX is *unavailable*
	[5/5]: split skiroot_defconfig changes out into its own patch

The whole Kconfig situation is really weird and confusing, I believe the
correct resolution is to change arch/Kconfig but the consequences are so
minor that I don't think it's worth it, especially given that I expect
powerpc to have mandatory strict RWX Soon(tm).

I'm not such strict RWX can be made mandatory due to the impact it has on
some subarches:
- On the 8xx, unless all areas are 8Mbytes aligned, there is a significant
overhead on TLB misses. And Aligning everthing to 8M is a waste of RAM which
is not acceptable on systems having very few RAM.
- On hash book3s32, we are able to map the kernel BATs. With a few alignment
constraints, we are able to provide STRICT_KERNEL_RWX. But we are unable to
provide exec protection on page granularity. Only on 256Mbytes segments. So
for modules, we have to have the vmspace X. It is also not possible to have
a kernel area RO. Only user areas can be made RO.

As I understand it, the idea was for it to be mandatory (or at least
default-on) only for the subarches where it wasn't totally insane to
accomplish. :) (I'm not familiar with all the details on the subarchs,
but it sounded like the more modern systems would be the targets for
this?)

Yes I guess so.

I was just afraid by "I expect powerpc to have mandatory strict RWX"

By the way, we have an open issue #223 namely 'Make strict kernel RWX 
the default on 64-bit', so no worry as 32-bit is aside for now.

Christophe

On Wed, 2019-10-30 at 09:58 +0100, Christophe Leroy wrote:

Le 30/10/2019 à 08:31, Russell Currey a écrit :

quoted

v4 cover letter: 
https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198268.html
v3 cover letter: 
https://lists.ozlabs.org/pipermail/linuxppc-dev/2019-October/198023.html

Changes since v4:
	[1/5]: Addressed review comments from Michael Ellerman
(thanks!)
	[4/5]: make ARCH_HAS_STRICT_MODULE_RWX depend on
	       ARCH_HAS_STRICT_KERNEL_RWX to simplify things and avoid
	       STRICT_MODULE_RWX being *on by default* in cases where
	       STRICT_KERNEL_RWX is *unavailable*
	[5/5]: split skiroot_defconfig changes out into its own patch

The whole Kconfig situation is really weird and confusing, I
believe the
correct resolution is to change arch/Kconfig but the consequences
are so
minor that I don't think it's worth it, especially given that I
expect
powerpc to have mandatory strict RWX Soon(tm).

I'm not such strict RWX can be made mandatory due to the impact it
has 
on some subarches:
- On the 8xx, unless all areas are 8Mbytes aligned, there is a 
significant overhead on TLB misses. And Aligning everthing to 8M is
a 
waste of RAM which is not acceptable on systems having very few RAM.
- On hash book3s32, we are able to map the kernel BATs. With a few 
alignment constraints, we are able to provide STRICT_KERNEL_RWX. But
we 
are unable to provide exec protection on page granularity. Only on 
256Mbytes segments. So for modules, we have to have the vmspace X. It
is 
also not possible to have a kernel area RO. Only user areas can be
made RO.

Yes, sorry, this was thoughtless from me, since in my mind I was just
thinking about the platforms I primarily work on (book3s64).

Christophe

quoted

Russell Currey (5):
   powerpc/mm: Implement set_memory() routines
   powerpc/kprobes: Mark newly allocated probes as RO
   powerpc/mm/ptdump: debugfs handler for W+X checks at runtime
   powerpc: Set ARCH_HAS_STRICT_MODULE_RWX
   powerpc/configs: Enable STRICT_MODULE_RWX in skiroot_defconfig

  arch/powerpc/Kconfig                   |  2 +
  arch/powerpc/Kconfig.debug             |  6 +-
  arch/powerpc/configs/skiroot_defconfig |  1 +
  arch/powerpc/include/asm/set_memory.h  | 32 +++++++++++
  arch/powerpc/kernel/kprobes.c          |  3 +
  arch/powerpc/mm/Makefile               |  1 +
  arch/powerpc/mm/pageattr.c             | 77
++++++++++++++++++++++++++
  arch/powerpc/mm/ptdump/ptdump.c        | 21 ++++++-
  8 files changed, 140 insertions(+), 3 deletions(-)
  create mode 100644 arch/powerpc/include/asm/set_memory.h
  create mode 100644 arch/powerpc/mm/pageattr.c