On 19 July 2018 at 00:33, Arnd Bergmann <arnd@xxxxxxxx> wrote: > On Wed, Jul 18, 2018 at 5:19 PM, Ard Biesheuvel > <ard.biesheuvel@xxxxxxxxxx> wrote: >> On 18 July 2018 at 23:50, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: >>> On 18 July 2018 at 05:59, Arnd Bergmann <arnd@xxxxxxxx> wrote: >>>> On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >>>>> >>>>> After my ahash to shash conversions, only ccm is left as an ahash >>>>> user, since it actually uses sg. But with the hard-coded value reduced >>>>> to 376, this doesn't trip the frame warnings any more. :) >>>>> >>>>> I'll send an updated series soon. >>>> >>>> Maybe we should get rid of that one as well then and remove >>>> AHASH_REQUEST_ON_STACK()? >>>> >>>> I see that Ard (now on Cc) added this usage only recently. Looking >>>> at the code some more, I also find that the descsize is probably >>>> much smaller than 376 for all possible cases of "cbcmac(*)", >>>> either alg->cra_blocksize plus a few bytes or sizeof(mac_desc_ctx) >>>> (i.e. 20) for arch/arm64/crypto/aes-glue.c. >>>> >>>> Walking the sglist here means open-coding a shash_ahash_update() >>>> implementation in crypto_ccm_auth(), that that doesn't seem to >>>> add much complexity over what it already has to do to chain >>>> the sglist today. >>>> >>> >>> It would be better to add a variably sized ahash request member to >>> struct crypto_ccm_req_priv_ctx, the only problem is that the last >>> member of that struct (skreq) is variably sized already, so it would >>> involve having a struct ahash_request pointer pointing into the same >>> struct, after the skreq member. >> >> Actually, I think the below should already do the trick: ahreq and >> skreq are not used at the same time, so we can stick them in a union, >> and take the max() of the reqsize to ensure there's enough empty space >> after it. > > This looks very nice indeed. > >> --------8<---------- >> diff --git a/crypto/ccm.c b/crypto/ccm.c >> index 0a083342ec8c..b242fd0d3262 100644 >> --- a/crypto/ccm.c >> +++ b/crypto/ccm.c >> @@ -50,7 +50,10 @@ struct crypto_ccm_req_priv_ctx { >> u32 flags; >> struct scatterlist src[3]; >> struct scatterlist dst[3]; >> - struct skcipher_request skreq; >> + union { >> + struct ahash_request ahreq; >> + struct skcipher_request skreq; >> + }; >> }; >> > > And this structure is never put on the stack anywhere but > always dynamically allocated anyway, right? > Yes.