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Subject[kernel-hardening][PATCH v3 3/3] arm: mm: dump: add checking for writable and executable pages
Page mappings with full RWX permissions are a security risk.
x86, arm64 has an option to walk the page tables
and dump any bad pages.

(1404d6f13e47
("arm64: dump: Add checking for writable and exectuable pages"))
Add a similar implementation for arm.

Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
---
v3: Reuse pg_level, prot_bits to check ro, nx prot.

arch/arm/Kconfig.debug | 27 +++++++++++++++++++++++
arch/arm/include/asm/ptdump.h | 8 +++++++
arch/arm/mm/dump.c | 51 +++++++++++++++++++++++++++++++++++++++++++
arch/arm/mm/init.c | 2 ++
4 files changed, 88 insertions(+)

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index e7b94db..78a6470 100644
--- a/arch/arm/Kconfig.debug
+++ b/arch/arm/Kconfig.debug
@@ -20,6 +20,33 @@ config ARM_PTDUMP_DEBUGFS
kernel.
If in doubt, say "N"

+config DEBUG_WX
+ bool "Warn on W+X mappings at boot"
+ select ARM_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.
+
+ Look for a message in dmesg output like this:
+
+ arm/mm: Checked W+X mappings: passed, no W+X pages found.
+
+ or like this, if the check failed:
+
+ arm/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".
+
# RMK wants arm kernels compiled with frame pointers or stack unwinding.
# If you know what you are doing and are willing to live without stack
# traces, you can get a slightly smaller kernel by setting this option to
diff --git a/arch/arm/include/asm/ptdump.h b/arch/arm/include/asm/ptdump.h
index 3a6c0b7..b6a0162 100644
--- a/arch/arm/include/asm/ptdump.h
+++ b/arch/arm/include/asm/ptdump.h
@@ -43,6 +43,14 @@ static inline int ptdump_debugfs_register(struct ptdump_info *info,
}
#endif /* CONFIG_ARM_PTDUMP_DEBUGFS */

+void ptdump_check_wx(void);
+
#endif /* CONFIG_ARM_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/arm/mm/dump.c b/arch/arm/mm/dump.c
index 43a2bee..3e2e6f0 100644
--- a/arch/arm/mm/dump.c
+++ b/arch/arm/mm/dump.c
@@ -52,6 +52,8 @@ struct pg_state {
unsigned long start_address;
unsigned level;
u64 current_prot;
+ bool check_wx;
+ unsigned long wx_pages;
const char *current_domain;
};

@@ -194,6 +196,8 @@ struct pg_level {
const struct prot_bits *bits;
size_t num;
u64 mask;
+ const struct prot_bits *ro_bit;
+ const struct prot_bits *nx_bit;
};

static struct pg_level pg_level[] = {
@@ -203,9 +207,17 @@ struct pg_level {
}, { /* pmd */
.bits = section_bits,
.num = ARRAY_SIZE(section_bits),
+ #ifdef CONFIG_ARM_LPAE
+ .ro_bit = section_bits + 1,
+ #else
+ .ro_bit = section_bits,
+ #endif
+ .nx_bit = section_bits + ARRAY_SIZE(section_bits) - 2,
}, { /* pte */
.bits = pte_bits,
.num = ARRAY_SIZE(pte_bits),
+ .ro_bit = pte_bits + 1,
+ .nx_bit = pte_bits + 2,
},
};

@@ -226,6 +238,23 @@ static void dump_prot(struct pg_state *st, const struct prot_bits *bits, size_t
}
}

+static void note_prot_wx(struct pg_state *st, unsigned long addr)
+{
+ if (!st->check_wx)
+ return;
+ if ((st->current_prot & pg_level[st->level].ro_bit->mask) ==
+ pg_level[st->level].ro_bit->val)
+ return;
+ if ((st->current_prot & pg_level[st->level].nx_bit->mask) ==
+ pg_level[st->level].nx_bit->val)
+ return;
+
+ WARN_ONCE(1, "arm/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 int level, u64 val, const char *domain)
{
@@ -244,6 +273,7 @@ static void note_page(struct pg_state *st, unsigned long addr,
unsigned long delta;

if (st->current_prot) {
+ note_prot_wx(st, addr);
pt_dump_seq_printf(st->seq, "0x%08lx-0x%08lx ",
st->start_address, addr);

@@ -367,6 +397,7 @@ void ptdump_walk_pgd(struct seq_file *m, struct ptdump_info *info)
struct pg_state st = {
.seq = m,
.marker = info->markers,
+ .check_wx = false,
};

walk_pgd(&st, info->mm, info->base_addr);
@@ -391,6 +422,26 @@ static void ptdump_initialize(void)
.base_addr = 0,
};

+void ptdump_check_wx(void)
+{
+ struct pg_state st = {
+ .seq = NULL,
+ .marker = (struct addr_marker[]) {
+ { 0, NULL},
+ { -1, NULL},
+ },
+ .check_wx = true,
+ };
+
+ walk_pgd(&st, &init_mm, 0);
+ note_page(&st, 0, 0, 0, NULL);
+ if (st.wx_pages)
+ pr_warn("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/arm/mm/init.c b/arch/arm/mm/init.c
index a1f11a7..bd6f451 100644
--- a/arch/arm/mm/init.c
+++ b/arch/arm/mm/init.c
@@ -36,6 +36,7 @@
#include <asm/system_info.h>
#include <asm/tlb.h>
#include <asm/fixmap.h>
+#include <asm/ptdump.h>

#include <asm/mach/arch.h>
#include <asm/mach/map.h>
@@ -738,6 +739,7 @@ static int __mark_rodata_ro(void *unused)
void mark_rodata_ro(void)
{
stop_machine(__mark_rodata_ro, NULL, NULL);
+ debug_checkwx();
}

void set_kernel_text_rw(void)
--
1.9.1
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