[PATCH 1/3] big key: get rid of stack array allocation

[Tycho Andersen]

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

Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: James Morris <jmorris@xxxxxxxxx>
CC: "Serge E. Hallyn" <serge@xxxxxxxxxx>
CC: Jason A. Donenfeld <Jason@xxxxxxxxx>
CC: Eric Biggers <ebiggers3@xxxxxxxxx>
---
 security/keys/big_key.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index 933623784ccd..75c46786a166 100644
--- a/security/keys/big_key.c
+++ b/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;
@@ -109,7 +110,7 @@ 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[GCM_AES_IV_SIZE];
 
 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
 	if (!aead_req)
@@ -425,6 +426,12 @@ 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) != GCM_AES_IV_SIZE)) {
+		WARN(1, "big key algorithm changed?");
+		return -EINVAL;
+	}
+
 	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);
-- 
2.17.0