Re: [PATCH v3 2/2] kasan, arm64: fix pointer tags in KASAN reports

[Vincenzo Frascino <vincenzo.frascino@arm.com>]

On 1/15/21 5:41 PM, Andrey Konovalov wrote:
> As of the "arm64: expose FAR_EL1 tag bits in siginfo" patch, the address
> that is passed to report_tag_fault has pointer tags in the format of 0x0X,
> while KASAN uses 0xFX format (note the difference in the top 4 bits).
> 
> Fix up the pointer tag for kernel pointers in do_tag_check_fault by
> setting them to the same value as bit 55. Explicitly use __untagged_addr()
> instead of untagged_addr(), as the latter doesn't affect TTBR1 addresses.
> 
> Link: https://linux-review.googlesource.com/id/I9ced973866036d8679e8f4ae325de547eb969649
> Fixes: dceec3ff7807 ("arm64: expose FAR_EL1 tag bits in siginfo")
> Fixes: 4291e9ee6189 ("kasan, arm64: print report from tag fault handler")
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>

> ---
>  arch/arm64/mm/fault.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 3c40da479899..35d75c60e2b8 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -709,10 +709,11 @@ static int do_tag_check_fault(unsigned long far, unsigned int esr,
>  			      struct pt_regs *regs)
>  {
>  	/*
> -	 * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN for tag
> -	 * check faults. Mask them out now so that userspace doesn't see them.
> +	 * The architecture specifies that bits 63:60 of FAR_EL1 are UNKNOWN
> +	 * for tag check faults. Set them to corresponding bits in the untagged
> +	 * address.
>  	 */
> -	far &= (1UL << 60) - 1;
> +	far = (__untagged_addr(far) & ~MTE_TAG_MASK) | (far & MTE_TAG_MASK);
>  	do_bad_area(far, esr, regs);
>  	return 0;
>  }
> 

-- 
Regards,
Vincenzo