Re: [PATCH] crypto: caam - Do not overwrite IV

[Herbert Xu <herbert@gondor.apana.org.au>]

On Mon, Feb 04, 2019 at 12:26:26PM +0000, Horia Geanta wrote:
>
> The root cause of the issue is cache line sharing.
> 
> struct crypto_gcm_req_priv_ctx {
>         u8 iv[16];
>         u8 auth_tag[16];
> 	[...]
> };
> 
> Since caam does not support ghash on i.MX6, only ctr skcipher part of the gcm is
> offloaded.
> The skcipher request received by caam has req->src pointing to auth_tag[16] (1st
> S/G entry) and req->iv pointing to iv[16].
> caam driver:
> 1-DMA maps req->src
> 2-copies original req->iv to internal buffer
> 3-updates req->iv (scatterwalk_map_and_copy from last block in req->src)
> 4-sends job to crypto engine
> 
> Problem is that operation 3 above is writing iv[16], which is on the same cache
> line as auth_tag[16] that was previously DMA mapped.
> 
> I've checked that forcing auth_tag and iv to be on separate cache lines
> -       u8 auth_tag[16];
> +       u8 auth_tag[16] ____cacheline_aligned;
> solves the issue.
> 
> OTOH, maybe the fix should be done in caam driver, by avoiding any writes
> (touching any data, even seemingly unrelated req->iv) after DMA mapping
> req->src, req->dst etc.
> Having req->iv and req->src sharing the same cache line is unfortunate.
> 
> Herbert, what do you think?

Well just like the other cases if your input is a kernel pointer you
must not perform DMA on it.  Only SG lists can be used for DMA.

So the IV needs to be copied on completion.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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