[PATCH v4 12/14] crypto: ahash: Remove VLA usage for AHASH_REQUEST_ON_STACK

[Kees Cook]

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 very large.
Looking at instrumented tcrypt output, the largest is for sha512:

	crypt: testing sha512
	crypto_ahash_set_reqsize: 528
	crypto_ahash_set_reqsize: 728
	crypto_ahash_set_reqsize: 808

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

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 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 4fcd0e2368cd..9290120536cd 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	808
 
 #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