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SubjectRe: [PATCHv3 4/4] arm64: dump: Add checking for writable and exectuable pages
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On 10/20/2016 03:32 AM, Ard Biesheuvel wrote:
> On 18 October 2016 at 23:01, Laura Abbott <labbott@redhat.com> 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@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>
>> ---
>> 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?
>
>

UXN is a separate bit from PTE_PXN/PTE_RDONLY and both pairs need to
be checked. The current return == 0 logic means that one set or the
other may not get checked. Rather than complicate the logic, it seemed
better to have separate functions. I see your point about the wx_pages
double counting so I can fix that.

>> 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?
>

Sure

>> + 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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