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From:   Kees Cook <keescook@chromium.org>
Date:   Wed, 27 Jun 2018 11:31:09 -0700
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Subject: Re: [PATCH v2 11/11] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     "David S. Miller" <davem@davemloft.net>,
        linux-crypto <linux-crypto@vger.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Alasdair Kergon <agk@redhat.com>,
        Giovanni Cabiddu <giovanni.cabiddu@intel.com>,
        Lars Persson <larper@axis.com>,
        Mike Snitzer <snitzer@redhat.com>,
        Rabin Vincent <rabinv@axis.com>,
        Tim Chen <tim.c.chen@linux.intel.com>, qat-linux@intel.com,
        dm-devel@redhat.com, LKML <linux-kernel@vger.kernel.org>
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On Wed, Jun 27, 2018 at 7:36 AM, Herbert Xu <herbert@gondor.apana.org.au> wrote:
> On Tue, Jun 26, 2018 at 09:45:09AM -0700, Kees Cook wrote:
>>
>> Which are likely to be wrapped together? Should I take this to 512 or
>> something else?
>
> The situation is similar to ahash.  While they're using the same
> skcipher interface, the underlying algorithms must all be
> synchronous.  In fact, if they're not then they're buggy.
>
> Therefore it makes no sense to use the general skcipher request
> size as a threshold.  You should look at synchronous skcipher
> algorithms only.

I might be catching on... so from this list, I should only "count" the
synchronous ones as being wrappable? The skcipher list is actually
pretty short:

crypto/cryptd.c:        crypto_skcipher_set_reqsize(
crypto/cryptd.c-                tfm, sizeof(struct
cryptd_skcipher_request_ctx));

The above is, AIUI, unwrapped, so I only need to count sizeof(struct
cryptd_skcipher_request_ctx)?

These are "simple" wrappers:

crypto/lrw.c:   crypto_skcipher_set_reqsize(tfm,
crypto_skcipher_reqsize(cipher) +
crypto/lrw.c-                                    sizeof(struct rctx));

crypto/simd.c-  reqsize = sizeof(struct skcipher_request);
crypto/simd.c-  reqsize += crypto_skcipher_reqsize(&cryptd_tfm->base);
crypto/simd.c:  crypto_skcipher_set_reqsize(tfm, reqsize);

crypto/xts.c:   crypto_skcipher_set_reqsize(tfm,
crypto_skcipher_reqsize(child) +
crypto/xts.c-                                    sizeof(struct rctx));

But what are the "legitimate" existing crypto_skcipher_reqsize() values here?

These are "complex" wrappers, with cts even adding blocksize to the mix...

crypto/ctr.c-   align = crypto_skcipher_alignmask(tfm);
crypto/ctr.c-   align &= ~(crypto_tfm_ctx_alignment() - 1);
crypto/ctr.c-   reqsize = align + sizeof(struct crypto_rfc3686_req_ctx) +
crypto/ctr.c-             crypto_skcipher_reqsize(cipher);
crypto/ctr.c:   crypto_skcipher_set_reqsize(tfm, reqsize);

crypto/cts.c-   align = crypto_skcipher_alignmask(tfm);
crypto/cts.c-   bsize = crypto_skcipher_blocksize(cipher);
crypto/cts.c-   reqsize = ALIGN(sizeof(struct crypto_cts_reqctx) +
crypto/cts.c-                   crypto_skcipher_reqsize(cipher),
crypto/cts.c-                   crypto_tfm_ctx_alignment()) +
crypto/cts.c-             (align & ~(crypto_tfm_ctx_alignment() - 1)) + bsize;
crypto/cts.c-
crypto/cts.c:   crypto_skcipher_set_reqsize(tfm, reqsize);

What values might be expected here? It seems the entire blocksize
needs to be included as well...

-Kees

-- 
Kees Cook
Pixel Security