Re: [PATCH v2 11/11] crypto: skcipher: Remove VLA usage for SKCIPHER_REQUEST_ON_STACK

Kees Cook
2018-06-27T18:31:16Z

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