Re: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK

[Ard Biesheuvel]

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.

--------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;
+       };
 };

 struct cbcmac_tfm_ctx {
@@ -181,7 +184,7 @@
        struct crypto_ccm_req_priv_ctx *pctx = crypto_ccm_reqctx(req);
        struct crypto_aead *aead = crypto_aead_reqtfm(req);
        struct crypto_ccm_ctx *ctx = crypto_aead_ctx(aead);
-       AHASH_REQUEST_ON_STACK(ahreq, ctx->mac);
+       struct ahash_request *ahreq = &pctx->ahreq;
        unsigned int assoclen = req->assoclen;
        struct scatterlist sg[3];
        u8 *odata = pctx->odata;
@@ -427,7 +430,7 @@
        crypto_aead_set_reqsize(
                tfm,
                align + sizeof(struct crypto_ccm_req_priv_ctx) +
-               crypto_skcipher_reqsize(ctr));
+               max(crypto_ahash_reqsize(mac), crypto_skcipher_reqsize(ctr)));

        return 0;