Re: [PATCHv3 4/4] arm64: dump: Add checking for writable and exectuable pages

From: Ard Biesheuvel
Date: Thu Oct 20 2016 - 06:32:52 EST

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?


> 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?

> + 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
>