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 <keesc...@chromium.org>
Reviewed-by: Mark Rutland <mark.rutl...@arm.com>
Tested-by: Mark Rutland <mark.rutl...@arm.com>
Signed-off-by: Laura Abbott <labb...@redhat.com>
---
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;
+}
+
 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,
-- 
2.7.4

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