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SubjectRe: [PATCH v10 12/22] kasan, arm64: fix up fault handling logic
On Tue, Nov 06, 2018 at 06:30:27PM +0100, Andrey Konovalov wrote:
> show_pte in arm64 fault handling relies on the fact that the top byte of
> a kernel pointer is 0xff, which isn't always the case with tag-based
> KASAN.

That's for the TTBR1 check, right?

i.e. for the following to work:

if (addr >= VA_START)

... we need the tag bits to be an extension of bit 55...

>
> This patch resets the top byte in show_pte.
>
> Reviewed-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> ---
> arch/arm64/mm/fault.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 7d9571f4ae3d..d9a84d6f3343 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -32,6 +32,7 @@
> #include <linux/perf_event.h>
> #include <linux/preempt.h>
> #include <linux/hugetlb.h>
> +#include <linux/kasan.h>
>
> #include <asm/bug.h>
> #include <asm/cmpxchg.h>
> @@ -141,6 +142,8 @@ void show_pte(unsigned long addr)
> pgd_t *pgdp;
> pgd_t pgd;
>
> + addr = (unsigned long)kasan_reset_tag((void *)addr);

... but this ORs in (0xffUL << 56), which is not correct for addresses
which aren't TTBR1 addresses to begin with, where bit 55 is clear, and
throws away useful information.

We could use untagged_addr() here, but that wouldn't be right for
kernels which don't use TBI1, and we'd erroneously report addresses
under the TTBR1 range as being in the TTBR1 range.

I also see that the entry assembly for el{1,0}_{da,ia} clears the tag
for EL0 addresses.

So we could have:

static inline bool is_ttbr0_addr(unsigned long addr)
{
/* entry assembly clears tags for TTBR0 addrs */
return addr < TASK_SIZE_64;
}

static inline bool is_ttbr1_addr(unsigned long addr)
{
/* TTBR1 addresses may have a tag if HWKASAN is in use */
return arch_kasan_reset_tag(addr) >= VA_START;
}
... and use those in the conditionals, leaving the addr as-is for
reporting purposes.

Thanks,
Mark.

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