On Tue, Jul 17, 2018 at 01:07:50PM -0700, Kees Cook wrote: > On Tue, Jul 17, 2018 at 9:39 AM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > > On Mon, Jul 16, 2018 at 09:21:44PM -0700, Kees Cook wrote: > >> In the quest to remove all stack VLA usage from the kernel[1], this > >> introduces max size macros for ahash, as already done for shash, and > >> adjust the crypto user to max state size. > >> > >> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx > >> > >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > >> --- > >> crypto/ahash.c | 4 ++-- > >> crypto/algif_hash.c | 2 +- > >> include/crypto/hash.h | 3 +++ > >> 3 files changed, 6 insertions(+), 3 deletions(-) > >> > >> diff --git a/crypto/ahash.c b/crypto/ahash.c > >> index a64c143165b1..6435bdbe42fd 100644 > >> --- a/crypto/ahash.c > >> +++ b/crypto/ahash.c > >> @@ -550,8 +550,8 @@ static int ahash_prepare_alg(struct ahash_alg *alg) > >> { > >> struct crypto_alg *base = &alg->halg.base; > >> > >> - if (alg->halg.digestsize > PAGE_SIZE / 8 || > >> - alg->halg.statesize > PAGE_SIZE / 8 || > >> + if (alg->halg.digestsize > AHASH_MAX_DIGESTSIZE || > >> + alg->halg.statesize > AHASH_MAX_STATESIZE || > >> alg->halg.statesize == 0) > >> return -EINVAL; > >> > >> diff --git a/crypto/algif_hash.c b/crypto/algif_hash.c > >> index bfcf595fd8f9..8974ee8ebead 100644 > >> --- a/crypto/algif_hash.c > >> +++ b/crypto/algif_hash.c > >> @@ -239,7 +239,7 @@ static int hash_accept(struct socket *sock, struct socket *newsock, int flags, > >> struct alg_sock *ask = alg_sk(sk); > >> struct hash_ctx *ctx = ask->private; > >> struct ahash_request *req = &ctx->req; > >> - char state[crypto_ahash_statesize(crypto_ahash_reqtfm(req)) ? : 1]; > >> + char state[AHASH_MAX_STATESIZE]; > >> struct sock *sk2; > >> struct alg_sock *ask2; > >> struct hash_ctx *ctx2; > >> diff --git a/include/crypto/hash.h b/include/crypto/hash.h > >> index ae14cc0e0cdb..4fcd0e2368cd 100644 > >> --- a/include/crypto/hash.h > >> +++ b/include/crypto/hash.h > >> @@ -64,6 +64,9 @@ struct ahash_request { > >> void *__ctx[] CRYPTO_MINALIGN_ATTR; > >> }; > >> > >> +#define AHASH_MAX_DIGESTSIZE 512 > >> +#define AHASH_MAX_STATESIZE 512 > >> + > > > > Why is AHASH_MAX_DIGESTSIZE (512) so much larger than SHASH_MAX_DIGESTSIZE (64)? > > I would have expected them to be the same. > > This was a direct replacement of the PAGE_SIZE / 8 original limit. > This this didn't trip any frame size warnings, it seemed like we > didn't need to do the more narrow limitations done elsewhere. > > I am, of course, happy to do a manual review and find a lower > alternative if that's desired. > I just don't see why ahash algorithms would need such a huge maximum digest size. Don't the 'ahash' algorithms all have 'shash' equivalents too? Is there actually any hash algorithm, either shash or ahash, in the Linux kernel that has a digest size greater than 64 bytes (512 bits)? Note that for a real cryptographic hash there isn't really any need for a digest size larger than that, since that already gives you 256-bit collision resistance; that's why SHA-2 and SHA-3 max out at that size. - Eric
