Re: [PATCH net-next] af_unix: fix use after free in unix_stream_recvmsg()

Le vendredi 09 septembre 2011 à 08:51 +0200, Eric Dumazet a écrit :

quoted

Now we have to fix a bug in unix_stream_recvmsg() as well.

consume_skb() call actually releases pid/cred references, and we can use
them after their eventual freeing.

Keep also in mind that receiver can provides a too short user buffer,
and skb can be put back to head of sk_receive_queue

Here is the patch to address this point.

Apply it after (af_unix: Fix use-after-free crashes)

I can make a combo patch once everybody agrees.

[PATCH net-next] af_unix: fix use after free in unix_stream_recvmsg()

Commit 0856a30409 (Scm: Remove unnecessary pid & credential references
in Unix socket's send and receive path) introduced an use-after-free
bug in unix_stream_recvmsg().

We should call consume_skb(skb) only after our possible use of pid/cred.

Signed-off-by: Eric Dumazet <redacted>
---
 net/unix/af_unix.c |   13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index c8a08ba..1bd4ecf 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -1873,6 +1873,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 	int target;
 	int err = 0;
 	long timeo;
+	struct sk_buff *skb;
 
 	err = -EINVAL;
 	if (sk->sk_state != TCP_ESTABLISHED)

@@ -1904,7 +1905,6 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 
 	do {
 		int chunk;
-		struct sk_buff *skb;
 
 		unix_state_lock(sk);
 		skb = skb_dequeue(&sk->sk_receive_queue);

@@ -1949,6 +1949,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 			if ((UNIXCB(skb).pid  != siocb->scm->pid) ||
 			    (UNIXCB(skb).cred != siocb->scm->cred)) {
 				skb_queue_head(&sk->sk_receive_queue, skb);
+				skb = NULL;
 				break;
 			}
 		} else {

@@ -1967,6 +1968,7 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 		chunk = min_t(unsigned int, skb->len, size);
 		if (memcpy_toiovec(msg->msg_iov, skb->data, chunk)) {
 			skb_queue_head(&sk->sk_receive_queue, skb);
+			skb = NULL;
 			if (copied == 0)
 				copied = -EFAULT;
 			break;

@@ -1984,13 +1986,14 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 			/* put the skb back if we didn't use it up.. */
 			if (skb->len) {
 				skb_queue_head(&sk->sk_receive_queue, skb);
+				skb = NULL;
 				break;
 			}
 
-			consume_skb(skb);
-
-			if (siocb->scm->fp)
+			if (UNIXCB(skb).pid || siocb->scm->fp)
 				break;
+			consume_skb(skb);
+			skb = NULL;
 		} else {
 			/* It is questionable, see note in unix_dgram_recvmsg.
 			 */

@@ -1999,12 +2002,14 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
 
 			/* put message back and return */
 			skb_queue_head(&sk->sk_receive_queue, skb);
+			skb = NULL;
 			break;
 		}
 	} while (size);
 
 	mutex_unlock(&u->readlock);
 	scm_recv(sock, msg, siocb->scm, flags);
+	consume_skb(skb);
 out:
 	return copied ? : err;
 }