crypto: hash - prevent using keyed hashes without setting key

Currently, almost none of the keyed hash algorithms check whether a key
has been set before proceeding.  Some algorithms are okay with this and
will effectively just use a key of all 0's or some other bogus default.
However, others will severely break, as demonstrated using
"hmac(sha3-512-generic)", the unkeyed use of which causes a kernel crash
via a (potentially exploitable) stack buffer overflow.

A while ago, this problem was solved for AF_ALG by pairing each hash
transform with a 'has_key' bool.  However, there are still other places
in the kernel where userspace can specify an arbitrary hash algorithm by
name, and the kernel uses it as unkeyed hash without checking whether it
is really unkeyed.  Examples of this include:

    - KEYCTL_DH_COMPUTE, via the KDF extension
    - dm-verity
    - dm-crypt, via the ESSIV support
    - dm-integrity, via the "internal hash" mode with no key given
    - drbd (Distributed Replicated Block Device)

This bug is especially bad for KEYCTL_DH_COMPUTE as that requires no
privileges to call.

Fix the bug for all users by adding a flag CRYPTO_TFM_NEED_KEY to the
->crt_flags of each hash transform that indicates whether the transform
still needs to be keyed or not.  Then, make the hash init, import, and
digest functions return -ENOKEY if the key is still needed.

The new flag also replaces the 'has_key' bool which algif_hash was
previously using, thereby simplifying the algif_hash implementation.

Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 17 insertions, 5 deletions

diff --git a/crypto/ahash.c b/crypto/ahash.c
index d2c8895..266fc1d 100644
--- a/crypto/ahash.c
+++ b/crypto/ahash.c
@@ -193,11 +193,18 @@ int crypto_ahash_setkey(struct crypto_ahash *tfm, const u8 *key,
 			unsigned int keylen)
 {
 	unsigned long alignmask = crypto_ahash_alignmask(tfm);
+	int err;
 
 	if ((unsigned long)key & alignmask)
-		return ahash_setkey_unaligned(tfm, key, keylen);
+		err = ahash_setkey_unaligned(tfm, key, keylen);
+	else
+		err = tfm->setkey(tfm, key, keylen);
+
+	if (err)
+		return err;
 
-	return tfm->setkey(tfm, key, keylen);
+	crypto_ahash_clear_flags(tfm, CRYPTO_TFM_NEED_KEY);
+	return 0;
 }
 EXPORT_SYMBOL_GPL(crypto_ahash_setkey);
 
@@ -368,7 +375,12 @@ EXPORT_SYMBOL_GPL(crypto_ahash_finup);
 
 int crypto_ahash_digest(struct ahash_request *req)
 {
-	return crypto_ahash_op(req, crypto_ahash_reqtfm(req)->digest);
+	struct crypto_ahash *tfm = crypto_ahash_reqtfm(req);
+
+	if (crypto_ahash_get_flags(tfm) & CRYPTO_TFM_NEED_KEY)
+		return -ENOKEY;
+
+	return crypto_ahash_op(req, tfm->digest);
 }
 EXPORT_SYMBOL_GPL(crypto_ahash_digest);
 
@@ -450,7 +462,6 @@ static int crypto_ahash_init_tfm(struct crypto_tfm *tfm)
 	struct ahash_alg *alg = crypto_ahash_alg(hash);
 
 	hash->setkey = ahash_nosetkey;
-	hash->has_setkey = false;
 	hash->export = ahash_no_export;
 	hash->import = ahash_no_import;
 
@@ -465,7 +476,8 @@ static int crypto_ahash_init_tfm(struct crypto_tfm *tfm)
 
 	if (alg->setkey) {
 		hash->setkey = alg->setkey;
-		hash->has_setkey = true;
+		if (!(alg->halg.base.cra_flags & CRYPTO_ALG_OPTIONAL_KEY))
+			crypto_ahash_set_flags(hash, CRYPTO_TFM_NEED_KEY);
 	}
 	if (alg->export)
 		hash->export = alg->export;