Re: [PATCH v4 05/11] arm64: csum: Disable KASAN for do_csum()

[Mark Rutland]

On Tue, Apr 21, 2020 at 04:15:31PM +0100, Will Deacon wrote:
> do_csum() over-reads the source buffer and therefore abuses
> READ_ONCE_NOCHECK() to avoid tripping up KASAN. In preparation for
> READ_ONCE_NOCHECK() becoming a macro, and therefore losing its
> '__no_sanitize_address' annotation, just annotate do_csum() explicitly
> and fall back to normal loads.
> 
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

>From a functional perspective:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

I know that Robin had a concern w.r.t. how this would affect the
codegen, but I think we can follow that up after the series as a whole
is merged.

Thanks,
Mark.


> ---
>  arch/arm64/lib/csum.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/lib/csum.c b/arch/arm64/lib/csum.c
> index 60eccae2abad..78b87a64ca0a 100644
> --- a/arch/arm64/lib/csum.c
> +++ b/arch/arm64/lib/csum.c
> @@ -14,7 +14,11 @@ static u64 accumulate(u64 sum, u64 data)
>  	return tmp + (tmp >> 64);
>  }
>  
> -unsigned int do_csum(const unsigned char *buff, int len)
> +/*
> + * We over-read the buffer and this makes KASAN unhappy. Instead, disable
> + * instrumentation and call kasan explicitly.
> + */
> +unsigned int __no_sanitize_address do_csum(const unsigned char *buff, int len)
>  {
>  	unsigned int offset, shift, sum;
>  	const u64 *ptr;
> @@ -42,7 +46,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
>  	 * odd/even alignment, and means we can ignore it until the very end.
>  	 */
>  	shift = offset * 8;
> -	data = READ_ONCE_NOCHECK(*ptr++);
> +	data = *ptr++;
>  #ifdef __LITTLE_ENDIAN
>  	data = (data >> shift) << shift;
>  #else
> @@ -58,10 +62,10 @@ unsigned int do_csum(const unsigned char *buff, int len)
>  	while (unlikely(len > 64)) {
>  		__uint128_t tmp1, tmp2, tmp3, tmp4;
>  
> -		tmp1 = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
> -		tmp2 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 2));
> -		tmp3 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 4));
> -		tmp4 = READ_ONCE_NOCHECK(*(__uint128_t *)(ptr + 6));
> +		tmp1 = *(__uint128_t *)ptr;
> +		tmp2 = *(__uint128_t *)(ptr + 2);
> +		tmp3 = *(__uint128_t *)(ptr + 4);
> +		tmp4 = *(__uint128_t *)(ptr + 6);
>  
>  		len -= 64;
>  		ptr += 8;
> @@ -85,7 +89,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
>  		__uint128_t tmp;
>  
>  		sum64 = accumulate(sum64, data);
> -		tmp = READ_ONCE_NOCHECK(*(__uint128_t *)ptr);
> +		tmp = *(__uint128_t *)ptr;
>  
>  		len -= 16;
>  		ptr += 2;
> @@ -100,7 +104,7 @@ unsigned int do_csum(const unsigned char *buff, int len)
>  	}
>  	if (len > 0) {
>  		sum64 = accumulate(sum64, data);
> -		data = READ_ONCE_NOCHECK(*ptr);
> +		data = *ptr;
>  		len -= 8;
>  	}
>  	/*
> -- 
> 2.26.1.301.g55bc3eb7cb9-goog
>