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    SubjectRe: [PATCH v4 11/14] treewide: Prepare to remove VLA usage for AHASH_REQUEST_ON_STACK
    On 18 July 2018 at 23:50, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
    > On 18 July 2018 at 05:59, Arnd Bergmann <arnd@arndb.de> wrote:
    >> On Sun, Jul 15, 2018 at 6:28 AM, Kees Cook <keescook@chromium.org> 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;
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      Last update: 2018-07-18 17:20    [W:4.853 / U:0.056 seconds]
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