arm64: dump: Add checking for writable and exectuable pages

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@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Laura Abbott <labbott@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[catalin.marinas@arm.com: folded fix for KASan out of bounds from Mark Rutland]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/Kconfig.debug b/arch/arm64/Kconfig.debug
index 21a5b74096417798367d777fb357a56be22e7a9a..d1ebd46872fdf746b1f2c84e0af3959522051009 100644 (file)
--- 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 f72ee69ee6025774ad0c79c0388a2557e8b2e7d5..6afd8476c60c63b15bdb6b4a04510ed36a1c0ac6 100644 (file)
--- 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 bb366490c5f70a53884714229f659b6df2e46bd5..ca74a2aace425b95ed95ecf0e70a78188621004e 100644 (file)
--- a/arch/arm64/mm/dump.c
+++ b/arch/arm64/mm/dump.c
@@ -74,6 +74,9 @@ struct pg_state {
        unsigned long start_address;
        unsigned level;
        u64 current_prot;
+       bool check_wx;
+       unsigned long wx_pages;
+       unsigned long uxn_pages;
 };
 
 struct prot_bits {
@@ -202,6 +205,35 @@ 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->uxn_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 +251,8 @@ 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 +378,26 @@ 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[]) {
+                       { 0, NULL},
+                       { -1, NULL},
+               },
+               .check_wx = true,
+       };
+
+       walk_pgd(&st, &init_mm, 0);
+       note_page(&st, 0, 0, 0);
+       if (st.wx_pages || st.uxn_pages)
+               pr_warn("Checked W+X mappings: FAILED, %lu W+X pages found, %lu non-UXN pages found\n",
+                       st.wx_pages, st.uxn_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 dd5f12d0959e13f4408def863f8bafd2f92ba50c..17243e43184e9d34665bc3abb122724009881db4 100644 (file)
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -40,6 +40,7 @@
 #include <asm/tlb.h>
 #include <asm/memblock.h>
 #include <asm/mmu_context.h>
+#include <asm/ptdump.h>
 
 u64 idmap_t0sz = TCR_T0SZ(VA_BITS);
 
@@ -438,6 +439,8 @@ void mark_rodata_ro(void)
 
        /* flush the TLBs after updating live kernel mappings */
        flush_tlb_all();
+
+       debug_checkwx();
 }
 
 static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end,