Re: [PATCHv3 4/4] arm64: dump: Add checking for writable and exectuable pages
From: Laura Abbott
Date: Thu Oct 20 2016 - 09:01:41 EST
On 10/20/2016 03:32 AM, Ard Biesheuvel wrote:
On 18 October 2016 at 23:01, Laura Abbott <labbott@xxxxxxxxxx> wrote:
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.
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
Tested-by: Mark Rutland <mark.rutland@xxxxxxx>
Signed-off-by: Laura Abbott <labbott@xxxxxxxxxx>
---
v3: Rebased for header guard fixup, whitespace fixes
---
arch/arm64/Kconfig.debug | 29 +++++++++++++++++++++++
arch/arm64/include/asm/ptdump.h | 8 +++++++
arch/arm64/mm/dump.c | 52 +++++++++++++++++++++++++++++++++++++++++
arch/arm64/mm/mmu.c | 2 ++
4 files changed, 91 insertions(+)
diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index 21a5b74..d1ebd46 100644
--- a/arch/arm64/Kconfig.debug
+++ b/arch/arm64/Kconfig.debug
@@ -42,6 +42,35 @@ 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 f72ee69..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;
+}
+
Why are these separate functions, and why is wx_pages increased twice,
potentially?
Given how rare non-UXN kernel mappings should be, could we not just add
if ((st->current_prot & PTE_UXN) == 0)
WARN(xxx)
(without the _ONCE) to note_prot_wx(), and drop note_prot_uxn() entirely?
UXN is a separate bit from PTE_PXN/PTE_RDONLY and both pairs need to
be checked. The current return == 0 logic means that one set or the
other may not get checked. Rather than complicate the logic, it seemed
better to have separate functions. I see your point about the wx_pages
double counting so I can fix that.
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);
Could we upgrade this to pr_warn?
Sure
+ 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,
--
2.7.4