[PATCH] ecryptfs: avoid ctx initialization race

[Kees Cook <keescook@chromium.org>]

It might be possible for two callers to race the mutex lock after the
NULL ctx check. Instead, move the lock above the check so there isn't
the possibility of leaking a crypto ctx. Additionally, report the full
algo name when failing.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/ecryptfs/crypto.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/ecryptfs/crypto.c b/fs/ecryptfs/crypto.c
index d107576..c134346 100644
--- a/fs/ecryptfs/crypto.c
+++ b/fs/ecryptfs/crypto.c
@@ -618,27 +618,28 @@ int ecryptfs_init_crypt_ctx(struct ecryptfs_crypt_stat *crypt_stat)
 			"key_size_bits = [%zd]\n",
 			crypt_stat->cipher, (int)strlen(crypt_stat->cipher),
 			crypt_stat->key_size << 3);
+	mutex_lock(&crypt_stat->cs_tfm_mutex);
 	if (crypt_stat->tfm) {
 		rc = 0;
-		goto out;
+		goto out_unlock;
 	}
-	mutex_lock(&crypt_stat->cs_tfm_mutex);
 	rc = ecryptfs_crypto_api_algify_cipher_name(&full_alg_name,
 						    crypt_stat->cipher, "cbc");
 	if (rc)
 		goto out_unlock;
 	crypt_stat->tfm = crypto_alloc_ablkcipher(full_alg_name, 0, 0);
-	kfree(full_alg_name);
 	if (IS_ERR(crypt_stat->tfm)) {
 		rc = PTR_ERR(crypt_stat->tfm);
 		crypt_stat->tfm = NULL;
 		ecryptfs_printk(KERN_ERR, "cryptfs: init_crypt_ctx(): "
 				"Error initializing cipher [%s]\n",
-				crypt_stat->cipher);
-		goto out_unlock;
+				full_alg_name);
+		goto out_free;
 	}
 	crypto_ablkcipher_set_flags(crypt_stat->tfm, CRYPTO_TFM_REQ_WEAK_KEY);
 	rc = 0;
+out_free:
+	kfree(full_alg_name);
 out_unlock:
 	mutex_unlock(&crypt_stat->cs_tfm_mutex);
 out:
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security