linux-kernel - Re: [PATCH v4 05/11] arm64: csum: Disable KASAN for do

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Message-ID: <20200422094951.GA54428@lakrids.cambridge.arm.com>
Date:   Wed, 22 Apr 2020 10:49:52 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        kernel-team@...roid.com, Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH v4 05/11] arm64: csum: Disable KASAN for do_csum()

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@....com>
> Cc: Robin Murphy <robin.murphy@....com>
> Signed-off-by: Will Deacon <will@...nel.org>

>From a functional perspective:

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

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
>