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>

diff --git a/security/keys/big_key.c b/security/keys/big_key.c
index 9336237..2806e70 100644 (file)
--- 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;
@@ -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);