We're interested in getting rid of all of the stack allocated arrays in the
kernel [1]. This patch removes one in keys by switching to malloc/free.
Note that we use kzalloc, to avoid leaking the nonce. I'm not sure this is
really necessary, but extra paranoia seems prudent.

Manually tested using the program from the add_key man page to trigger
big_key.

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

Signed-off-by: Tycho Andersen <redacted>
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>
---
 security/keys/big_key.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index fa728f662a6f..70f9f785c59d 100644
--- a/security/keys/big_key.c
+++ b/security/keys/big_key.c

@@ -108,13 +108,18 @@ 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;
+
+	zero_nonce = kzalloc(crypto_aead_ivsize(big_key_aead), GFP_KERNEL);
+	if (!zero_nonce)
+		return -ENOMEM;
 
 	aead_req = aead_request_alloc(big_key_aead, GFP_KERNEL);
-	if (!aead_req)
+	if (!aead_req) {
+		kfree(zero_nonce);
 		return -ENOMEM;
+	}
 
-	memset(zero_nonce, 0, sizeof(zero_nonce));
 	aead_request_set_crypt(aead_req, buf->sg, buf->sg, datalen, zero_nonce);
 	aead_request_set_callback(aead_req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
 	aead_request_set_ad(aead_req, 0);

@@ -131,6 +136,7 @@ static int big_key_crypt(enum big_key_op op, struct big_key_buf *buf, size_t dat
 error:
 	mutex_unlock(&big_key_aead_lock);
 	aead_request_free(aead_req);
+	kzfree(zero_nonce);
 	return ret;