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 initialization. AHASH_REQUEST_ON_STACK is special, though: it
is only ever used for shash-wrapped ahash, so its size is bounded only
by non-async hashes. A manual inspection of this shows the largest to be:
sizeof(struct shash_desc) + SHASH_MAX_DESCSIZE

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

Signed-off-by: Kees Cook <redacted>
---
 crypto/shash.c        |  9 ++++++++-
 include/crypto/hash.h | 10 +++++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/crypto/shash.c b/crypto/shash.c
index 8d4746b14dd5..e344560458cb 100644
--- a/crypto/shash.c
+++ b/crypto/shash.c

@@ -355,6 +355,7 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
 	struct crypto_ahash *crt = __crypto_ahash_cast(tfm);
 	struct crypto_shash **ctx = crypto_tfm_ctx(tfm);
 	struct crypto_shash *shash;
+	size_t reqsize;
 
 	if (!crypto_mod_get(calg))
 		return -EAGAIN;

@@ -365,6 +366,12 @@ int crypto_init_shash_ops_async(struct crypto_tfm *tfm)
 		return PTR_ERR(shash);
 	}
 
+	reqsize = sizeof(struct shash_desc) + crypto_shash_descsize(shash);
+	if (WARN_ON(reqsize > AHASH_MAX_REQSIZE)) {
+		crypto_mod_put(calg);
+		return -EINVAL;
+	}