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From:   Ben Hutchings <ben@decadent.org.uk>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
CC:     akpm@linux-foundation.org, "Eric Biggers" <ebiggers@google.com>,
        "Mimi Zohar" <zohar@linux.vnet.ibm.com>,
        "David Howells" <dhowells@redhat.com>,
        "James Morris" <james.l.morris@oracle.com>
Date:   Sun, 09 Dec 2018 21:50:33 +0000
Message-ID: <lsq.1544392233.265897703@decadent.org.uk>
Subject: [PATCH 3.16 322/328] KEYS: encrypted: fix buffer overread in
 valid_master_desc()
In-Reply-To: <lsq.1544392232.713909046@decadent.org.uk>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

3.16.62-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Biggers <ebiggers@google.com>

commit 794b4bc292f5d31739d89c0202c54e7dc9bc3add upstream.

With the 'encrypted' key type it was possible for userspace to provide a
data blob ending with a master key description shorter than expected,
e.g. 'keyctl add encrypted desc "new x" @s'.  When validating such a
master key description, validate_master_desc() could read beyond the end
of the buffer.  Fix this by using strncmp() instead of memcmp().  [Also
clean up the code to deduplicate some logic.]

Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 security/keys/encrypted-keys/encrypted.c | 31 ++++++++++++------------
 1 file changed, 15 insertions(+), 16 deletions(-)

--- a/security/keys/encrypted-keys/encrypted.c
+++ b/security/keys/encrypted-keys/encrypted.c
@@ -141,23 +141,22 @@ static int valid_ecryptfs_desc(const cha
  */
 static int valid_master_desc(const char *new_desc, const char *orig_desc)
 {
-	if (!memcmp(new_desc, KEY_TRUSTED_PREFIX, KEY_TRUSTED_PREFIX_LEN)) {
-		if (strlen(new_desc) == KEY_TRUSTED_PREFIX_LEN)
-			goto out;
-		if (orig_desc)
-			if (memcmp(new_desc, orig_desc, KEY_TRUSTED_PREFIX_LEN))
-				goto out;
-	} else if (!memcmp(new_desc, KEY_USER_PREFIX, KEY_USER_PREFIX_LEN)) {
-		if (strlen(new_desc) == KEY_USER_PREFIX_LEN)
-			goto out;
-		if (orig_desc)
-			if (memcmp(new_desc, orig_desc, KEY_USER_PREFIX_LEN))
-				goto out;
-	} else
-		goto out;
+	int prefix_len;
+
+	if (!strncmp(new_desc, KEY_TRUSTED_PREFIX, KEY_TRUSTED_PREFIX_LEN))
+		prefix_len = KEY_TRUSTED_PREFIX_LEN;
+	else if (!strncmp(new_desc, KEY_USER_PREFIX, KEY_USER_PREFIX_LEN))
+		prefix_len = KEY_USER_PREFIX_LEN;
+	else
+		return -EINVAL;
+
+	if (!new_desc[prefix_len])
+		return -EINVAL;
+
+	if (orig_desc && strncmp(new_desc, orig_desc, prefix_len))
+		return -EINVAL;
+
 	return 0;
-out:
-	return -EINVAL;
 }
 
 /*