linux-kernel - Re: vmalloced stacks and scatterwalk_map_and

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Date:   Mon, 21 Nov 2016 10:08:14 -0800
From:   Eric Biggers <ebiggers@...gle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Andy Lutomirski <luto@...capital.net>, regressions@...mhuis.info,
        linux-crypto@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>
Subject: Re: vmalloced stacks and scatterwalk_map_and_copy()

On Mon, Nov 21, 2016 at 04:26:19PM +0800, Herbert Xu wrote:
> crypto: scatterwalk - Remove unnecessary aliasing check in map_and_copy
> 
> The aliasing check in map_and_copy is no longer necessary because
> the IPsec ESP code no longer provides an IV that points into the
> actual request data.  As this check is now triggering BUG checks
> due to the vmalloced stack code, I'm removing it.
> 
> Reported-by: Eric Biggers <ebiggers@...gle.com>
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> diff --git a/crypto/scatterwalk.c b/crypto/scatterwalk.c
> index 52ce17a..c16c94f8 100644
> --- a/crypto/scatterwalk.c
> +++ b/crypto/scatterwalk.c
> @@ -68,10 +68,6 @@ void scatterwalk_map_and_copy(void *buf, struct scatterlist *sg,
>  
>  	sg = scatterwalk_ffwd(tmp, sg, start);
>  
> -	if (sg_page(sg) == virt_to_page(buf) &&
> -	    sg->offset == offset_in_page(buf))
> -		return;
> -
>  	scatterwalk_start(&walk, sg);
>  	scatterwalk_copychunks(buf, &walk, nbytes, out);
>  	scatterwalk_done(&walk, out, 0);

This looks fine to me if you're confident that the aliasing check is indeed no
longer necessary.

Another idea I had was to replace memcpy() with memmove().  But I don't want to
be in a situation where we're stuck with memmove() forever because of users who
probably don't even exist.

Eric