linux-kernel - [PATCH] ecryptfs: avoid ctx initialization race

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Date:	Tue, 13 Aug 2013 15:02:27 -0700
From:	Kees Cook <keescook@...omium.org>
To:	linux-kernel@...r.kernel.org
Cc:	Tyler Hicks <tyhicks@...onical.com>, ecryptfs@...r.kernel.org
Subject: [PATCH] ecryptfs: avoid ctx initialization race

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@...omium.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
--
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