[PATCH v2 10/11] crypto: ahash: Remove VLA usage for AHASH_REQUEST_ON_STACK

Kees Cook
2018-06-25T21:11:46Z

In the quest to remove all stack VLA usage from the kernel[1], this caps
the ahash request size similar to the other limits and adds a sanity
check at registration. Unfortunately, these reqsizes can be huge. Looking
at all callers of crypto_ahash_set_reqsize(), the largest appears to be
664 bytes, based on a combination of manual inspection and pahole output:

4 dcp_sha_req_ctx
40 crypto4xx_ctx
52 hash_req_ctx
80 ahash_request
88 rk_ahash_rctx
104 sun4i_req_ctx
200 mcryptd_hash_request_ctx
216 safexcel_ahash_req
228 sha1_hash_ctx
228 sha256_hash_ctx
248 img_hash_request_ctx
252 mtk_sha_reqctx
276 sahara_sha_reqctx
304 mv_cesa_ahash_req
316 iproc_reqctx_s
320 caam_hash_state
320 qce_sha_reqctx
356 sha512_hash_ctx
384 ahash_req_ctx
400 chcr_ahash_req_ctx
416 stm32_hash_request_ctx
432 talitos_ahash_req_ctx
462 atmel_sha_reqctx
496 ccp_aes_cmac_req_ctx
616 ccp_sha_req_ctx
664 artpec6_hash_request_context

So, this chooses 720 as a larger "round" number for the max.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Biggers <ebiggers@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Rabin Vincent <rabinv@axis.com>
Cc: Lars Persson <larper@axis.com>
Cc: linux-crypto@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/crypto/hash.h | 3 ++-
include/crypto/internal/hash.h | 1 +
2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/crypto/hash.h b/include/crypto/hash.h
index 5d79e2f0936e..b550077c4767 100644
--- a/include/crypto/hash.h
+++ b/include/crypto/hash.h
@@ -66,10 +66,11 @@ struct ahash_request {

#define AHASH_MAX_DIGESTSIZE 512
#define AHASH_MAX_STATESIZE 512
+#define AHASH_MAX_REQSIZE 720

#define AHASH_REQUEST_ON_STACK(name, ahash) \
char __##name##_desc[sizeof(struct ahash_request) + \
- crypto_ahash_reqsize(ahash)] CRYPTO_MINALIGN_ATTR; \
+ AHASH_MAX_REQSIZE] CRYPTO_MINALIGN_ATTR; \
struct ahash_request *name = (void *)__##name##_desc

/**
diff --git a/include/crypto/internal/hash.h b/include/crypto/internal/hash.h
index a0b0ad9d585e..d96ae5f52125 100644
--- a/include/crypto/internal/hash.h
+++ b/include/crypto/internal/hash.h
@@ -142,6 +142,7 @@ static inline struct ahash_alg *__crypto_ahash_alg(struct crypto_alg *alg)
static inline void crypto_ahash_set_reqsize(struct crypto_ahash *tfm,
unsigned int reqsize)
{
+ BUG_ON(reqsize > AHASH_MAX_REQSIZE);
tfm->reqsize = reqsize;
}

--
2.17.1