fscrypt: verify that the crypto_skcipher has the correct ivsize

As a sanity check, verify that the allocated crypto_skcipher actually
has the ivsize that fscrypt is assuming it has.  This will always be the
case unless there's a bug.  But if there ever is such a bug (e.g. like
there was in earlier versions of the ESSIV conversion patch [1]) it's
preferable for it to be immediately obvious, and not rely on the
ciphertext verification tests failing due to uninitialized IV bytes.

[1] https://lkml.kernel.org/linux-crypto/20190702215517.GA69157@gmail.com/

Link: https://lore.kernel.org/r/20191209203918.225691-1-ebiggers@kernel.orgSigned-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/keysetup.c

@@ -91,6 +91,10 @@ struct crypto_skcipher *fscrypt_allocate_skcipher(struct fscrypt_mode *mode,
 		pr_info("fscrypt: %s using implementation \"%s\"\n",
 			mode->friendly_name, crypto_skcipher_driver_name(tfm));
 	}
+	if (WARN_ON(crypto_skcipher_ivsize(tfm) != mode->ivsize)) {
+		err = -EINVAL;
+		goto err_free_tfm;
+	}
 	crypto_skcipher_set_flags(tfm, CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
 	err = crypto_skcipher_setkey(tfm, raw_key, mode->keysize);
 	if (err)