big key: get rid of stack array allocation

We're interested in getting rid of all of the stack allocated arrays in the
kernel [1]. This patch simply hardcodes the iv length to match that of the
hardcoded cipher.

[1]: https://lkml.org/lkml/2018/3/7/621

v2: hardcode the length of the nonce to be the GCM AES IV length, and do a
    sanity check in init(), Eric Biggers
v3: * remember to free big_key_aead when sanity check fails
    * define a constant for big key IV size so it can be changed along side
      the algorithm in the code

Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Reviewed-by: Kees Cook <keescook@chromium.org>
CC: David Howells <dhowells@redhat.com>
CC: James Morris <jmorris@namei.org>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Jason A. Donenfeld <Jason@zx2c4.com>
CC: Eric Biggers <ebiggers3@gmail.com>
Signed-off-by: James Morris <james.morris@microsoft.com>

security/keys/big_key.c

@@ -22,6 +22,7 @@
 #include <keys/user-type.h>
 #include <keys/big_key-type.h>
 #include <crypto/aead.h>
+#include <crypto/gcm.h>
 
 struct big_key_buf {
 	unsigned int		nr_pages;
@@ -85,6 +86,7 @@ struct key_type key_type_big_key = {
  * Crypto names for big_key data authenticated encryption
  */
 static const char big_key_alg_name[] = "gcm(aes)";
+#define BIG_KEY_IV_SIZE		GCM_AES_IV_SIZE
 
 /*
  * Crypto algorithms for big_key data authenticated encryption
@@ -109,7 +111,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
 	 * an .update function, so there's no chance we'll wind up reusing the
 	 * key to encrypt updated data. Simply put: one key, one encryption.
 	 */
-	u8 zero_nonce[crypto_aead_ivsize(big_key_aead)];
+	u8 zero_nonce[BIG_KEY_IV_SIZE];
 
 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
 	if (!aead_req)
@@ -425,6 +427,13 @@ static int __init big_key_init(void)
 		pr_err("Can't alloc crypto: %d\n", ret);
 		return ret;
 	}
+
+	if (unlikely(crypto_aead_ivsize(big_key_aead) != BIG_KEY_IV_SIZE)) {
+		WARN(1, "big key algorithm changed?");
+		ret = -EINVAL;
+		goto free_aead;
+	}
+
 	ret = crypto_aead_setauthsize(big_key_aead, ENC_AUTHTAG_SIZE);
 	if (ret < 0) {
 		pr_err("Can't set crypto auth tag len: %d\n", ret);