ARM: 8737/1: mm: dump: add checking for writable and executable

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.

Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Jinbum Park <jinb.park7@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>

diff --git a/arch/arm/Kconfig.debug b/arch/arm/Kconfig.debug
index e7b94db..78a6470 100644 (file)
--- 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 45d2de3..3ebf971 100644 (file)
--- a/arch/arm/include/asm/ptdump.h
+++ b/arch/arm/include/asm/ptdump.h
@@ -30,6 +30,14 @@ 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..084779c 100644 (file)
--- 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;
 };
 
@@ -60,6 +62,8 @@ struct prot_bits {
        u64             val;
        const char      *set;
        const char      *clear;
+       bool            ro_bit;
+       bool            nx_bit;
 };
 
 static const struct prot_bits pte_bits[] = {
@@ -73,11 +77,13 @@ static const struct prot_bits pte_bits[] = {
                .val    = L_PTE_RDONLY,
                .set    = "ro",
                .clear  = "RW",
+               .ro_bit = true,
        }, {
                .mask   = L_PTE_XN,
                .val    = L_PTE_XN,
                .set    = "NX",
                .clear  = "x ",
+               .nx_bit = true,
        }, {
                .mask   = L_PTE_SHARED,
                .val    = L_PTE_SHARED,
@@ -141,11 +147,13 @@ static const struct prot_bits section_bits[] = {
                .val    = L_PMD_SECT_RDONLY | PMD_SECT_AP2,
                .set    = "ro",
                .clear  = "RW",
+               .ro_bit = true,
 #elif __LINUX_ARM_ARCH__ >= 6
        {
                .mask   = PMD_SECT_APX | PMD_SECT_AP_READ | PMD_SECT_AP_WRITE,
                .val    = PMD_SECT_APX | PMD_SECT_AP_WRITE,
                .set    = "    ro",
+               .ro_bit = true,
        }, {
                .mask   = PMD_SECT_APX | PMD_SECT_AP_READ | PMD_SECT_AP_WRITE,
                .val    = PMD_SECT_AP_WRITE,
@@ -164,6 +172,7 @@ static const struct prot_bits section_bits[] = {
                .mask   = PMD_SECT_AP_READ | PMD_SECT_AP_WRITE,
                .val    = 0,
                .set    = "    ro",
+               .ro_bit = true,
        }, {
                .mask   = PMD_SECT_AP_READ | PMD_SECT_AP_WRITE,
                .val    = PMD_SECT_AP_WRITE,
@@ -182,6 +191,7 @@ static const struct prot_bits section_bits[] = {
                .val    = PMD_SECT_XN,
                .set    = "NX",
                .clear  = "x ",
+               .nx_bit = true,
        }, {
                .mask   = PMD_SECT_S,
                .val    = PMD_SECT_S,
@@ -194,6 +204,8 @@ 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[] = {
@@ -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 %pS\n",
+                       (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);
@@ -379,8 +410,13 @@ static void ptdump_initialize(void)
 
        for (i = 0; i < ARRAY_SIZE(pg_level); i++)
                if (pg_level[i].bits)
-                       for (j = 0; j < pg_level[i].num; j++)
+                       for (j = 0; j < pg_level[i].num; j++) {
                                pg_level[i].mask |= pg_level[i].bits[j].mask;
+                               if (pg_level[i].bits[j].ro_bit)
+                                       pg_level[i].ro_bit = &pg_level[i].bits[j];
+                               if (pg_level[i].bits[j].nx_bit)
+                                       pg_level[i].nx_bit = &pg_level[i].bits[j];
+                       }
 
        address_markers[2].start_address = VMALLOC_START;
 }
@@ -391,6 +427,26 @@ static struct ptdump_info kernel_ptdump_info = {
        .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 (file)
--- 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)