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SubjectRe: [PATCH 1/2] big key: get rid of stack array allocation
On Mon, Mar 12, 2018 at 10:29:06PM -0600, Tycho Andersen wrote:
> 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 <tycho@tycho.ws>
> 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;

A dynamic allocation here doesn't make sense -- the algorithm is hard-coded to
AES-GCM, so the IV size is fixed. You should just include <crypto/gcm.h> and
use GCM_AES_IV_LEN. As a sanity check you can add
'BUG_ON(crypto_aead_ivsize(big_key_aead) != GCM_AES_IV_LEN' to big_key_init().

kzfree() also doesn't make sense since the nonce is not secret information.

Thanks,

Eric

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