[PATCHv2,4/4] arm64: dump: Add checking for writable and exectuable pages
diff mbox

Message ID 1476311522-15381-5-git-send-email-labbott@redhat.com
State New
Headers show

Commit Message

Laura Abbott Oct. 12, 2016, 10:32 p.m. UTC
Page mappings with full RWX permissions are a security risk. x86
has an option to walk the page tables and dump any bad pages.
(See e1a58320a38d ("x86/mm: Warn on W^X mappings")). Add a similar
implementation for arm64.

Signed-off-by: Laura Abbott <labbott@redhat.com>
---
v2: Check only init_mm, style cleanups, UXN checks, compiliation fixes for
    disabled case.
---
 arch/arm64/Kconfig.debug        | 30 ++++++++++++++++++++++++
 arch/arm64/include/asm/ptdump.h |  8 +++++++
 arch/arm64/mm/dump.c            | 52 +++++++++++++++++++++++++++++++++++++++++
 arch/arm64/mm/mmu.c             |  2 ++
 4 files changed, 92 insertions(+)

Comments

Mark Rutland Oct. 17, 2016, 12:47 p.m. UTC | #1
On Wed, Oct 12, 2016 at 03:32:02PM -0700, Laura Abbott wrote:
> +config DEBUG_WX
> +	bool "Warn on W+X mappings at boot"
> +	select ARM64_PTDUMP_CORE
> +	---help---
> +	  Generate a warning if any W+X mappings are found at boot.
> +
> +	  This is useful for discovering cases where the kernel is leaving
> +	  W+X mappings after applying NX, as such mappings are a security risk.
> +	  This check also includes UXN, which should be set on all kernel
> +	  mappings.
> +
> +	  Look for a message in dmesg output like this:
> +
> +	    arm64/mm: Checked W+X mappings: passed, no W+X pages found.
> +
> +	  or like this, if the check failed:
> +
> +	    arm64/mm: Checked W+X mappings: FAILED, <N> W+X pages found.
> +
> +	  Note that even if the check fails, your kernel is possibly
> +	  still fine, as W+X mappings are not a security hole in
> +	  themselves, what they do is that they make the exploitation
> +	  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.
> +
> +	  If in doubt, say "Y".
> +
> +

Trivial nit: for consistency with the rest of the file, there should
only be one line space between options.

>  config DEBUG_SET_MODULE_RONX
>  	bool "Set loadable kernel module data as NX and text as RO"
>  	depends on MODULES
> diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
> index 8fc0957..6afd847 100644
> --- a/arch/arm64/include/asm/ptdump.h
> +++ b/arch/arm64/include/asm/ptdump.h
> @@ -42,5 +42,13 @@ static inline int ptdump_debugfs_register(struct ptdump_info *info,
>  	return 0;
>  }
>  #endif
> +void ptdump_check_wx(void);
> +#endif /* CONFIG_ARM64_PTDUMP_CORE */

... ah, here's the missing #endif from patch 1.

Assuming that gets sorted out, this looks good to me, and works on juno,
so FWIW:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>

[...]

> +static void note_prot_uxn(struct pg_state *st, unsigned long addr)
> +{
> +	if (!st->check_wx)
> +		return;
> +
> +	if ((st->current_prot & PTE_UXN) == PTE_UXN)
> +		return;
> +
> +	WARN_ONCE(1, "arm64/mm: Found non-UXN mapping at address %p/%pS\n",
> +		  (void *)st->start_address, (void *)st->start_address);
> +
> +	st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
> +}

As a future thought (I've scope-creeped this enough with the UXN check),
there are some other checks that we could add, like verifying the AP
bits don't allow user data access. That might be worth considering, with
DEBUG_WX becoming a more general kernel page table sanity check.

Thanks,
Mark.

Patch
diff mbox

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index 21a5b74..3f627c94 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -42,6 +42,36 @@  config ARM64_RANDOMIZE_TEXT_OFFSET
 	  of TEXT_OFFSET and platforms must not require a specific
 	  value.
 
+config DEBUG_WX
+	bool "Warn on W+X mappings at boot"
+	select ARM64_PTDUMP_CORE
+	---help---
+	  Generate a warning if any W+X mappings are found at boot.
+
+	  This is useful for discovering cases where the kernel is leaving
+	  W+X mappings after applying NX, as such mappings are a security risk.
+	  This check also includes UXN, which should be set on all kernel
+	  mappings.
+
+	  Look for a message in dmesg output like this:
+
+	    arm64/mm: Checked W+X mappings: passed, no W+X pages found.
+
+	  or like this, if the check failed:
+
+	    arm64/mm: Checked W+X mappings: FAILED, <N> W+X pages found.
+
+	  Note that even if the check fails, your kernel is possibly
+	  still fine, as W+X mappings are not a security hole in
+	  themselves, what they do is that they make the exploitation
+	  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.
+
+	  If in doubt, say "Y".
+
+
 config DEBUG_SET_MODULE_RONX
 	bool "Set loadable kernel module data as NX and text as RO"
 	depends on MODULES
diff --git a/arch/arm64/include/asm/ptdump.h b/arch/arm64/include/asm/ptdump.h
index 8fc0957..6afd847 100644
--- a/arch/arm64/include/asm/ptdump.h
+++ b/arch/arm64/include/asm/ptdump.h
@@ -42,5 +42,13 @@  static inline int ptdump_debugfs_register(struct ptdump_info *info,
 	return 0;
 }
 #endif
+void ptdump_check_wx(void);
+#endif /* CONFIG_ARM64_PTDUMP_CORE */
+
+#ifdef CONFIG_DEBUG_WX
+#define debug_checkwx()	ptdump_check_wx()
+#else
+#define debug_checkwx()	do { } while (0)
+#endif
 
 #endif /* __ASM_PTDUMP_H */
diff --git a/arch/arm64/mm/dump.c b/arch/arm64/mm/dump.c
index bb36649..4913af5 100644
--- a/arch/arm64/mm/dump.c
+++ b/arch/arm64/mm/dump.c
@@ -74,6 +74,8 @@  struct pg_state {
 	unsigned long start_address;
 	unsigned level;
 	u64 current_prot;
+	bool check_wx;
+	unsigned long wx_pages;
 };
 
 struct prot_bits {
@@ -202,6 +204,35 @@  static void dump_prot(struct pg_state *st, const struct prot_bits *bits,
 	}
 }
 
+static void note_prot_uxn(struct pg_state *st, unsigned long addr)
+{
+	if (!st->check_wx)
+		return;
+
+	if ((st->current_prot & PTE_UXN) == PTE_UXN)
+		return;
+
+	WARN_ONCE(1, "arm64/mm: Found non-UXN mapping at address %p/%pS\n",
+		  (void *)st->start_address, (void *)st->start_address);
+
+	st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
+}
+
+static void note_prot_wx(struct pg_state *st, unsigned long addr)
+{
+	if (!st->check_wx)
+		return;
+	if ((st->current_prot & PTE_RDONLY) == PTE_RDONLY)
+		return;
+	if ((st->current_prot & PTE_PXN) == PTE_PXN)
+		return;
+
+	WARN_ONCE(1, "arm64/mm: Found insecure W+X mapping at address %p/%pS\n",
+		  (void *)st->start_address, (void *)st->start_address);
+
+	st->wx_pages += (addr - st->start_address) / PAGE_SIZE;
+}
+
 static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
 				u64 val)
 {
@@ -219,6 +250,8 @@  static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
 		unsigned long delta;
 
 		if (st->current_prot) {
+			note_prot_uxn(st, addr);
+			note_prot_wx(st, addr);
 			pt_dump_seq_printf(st->seq, "0x%016lx-0x%016lx   ",
 				   st->start_address, addr);
 
@@ -344,6 +377,25 @@  static struct ptdump_info kernel_ptdump_info = {
 	.base_addr	= VA_START,
 };
 
+void ptdump_check_wx(void)
+{
+	struct pg_state st = {
+		.seq = NULL,
+		.marker = (struct addr_marker[]) {
+			{ -1, NULL},
+		},
+		.check_wx = true,
+	};
+
+	walk_pgd(&st, &init_mm, 0);
+	note_page(&st, 0, 0, 0);
+	if (st.wx_pages)
+		pr_info("Checked W+X mappings: FAILED, %lu W+X pages found\n",
+			st.wx_pages);
+	else
+		pr_info("Checked W+X mappings: passed, no W+X pages found\n");
+}
+
 static int ptdump_init(void)
 {
 	ptdump_initialize();
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 05615a3..2cbe2fe 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -42,6 +42,7 @@ 
 #include <asm/tlb.h>
 #include <asm/memblock.h>
 #include <asm/mmu_context.h>
+#include <asm/ptdump.h>
 
 u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
 
@@ -396,6 +397,7 @@  void mark_rodata_ro(void)
 	section_size = (unsigned long)__init_begin - (unsigned long)__start_rodata;
 	create_mapping_late(__pa(__start_rodata), (unsigned long)__start_rodata,
 			    section_size, PAGE_KERNEL_RO);
+	debug_checkwx();
 }
 
 static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,