[PATCH net-next v3] af_unix: Fix use-after-free crashes

[Eric Dumazet <eric.dumazet@gmail.com>]

Le jeudi 08 septembre 2011 à 11:59 +0200, Sedat Dilek a écrit :

> I have tested this fixup patch on i386.
> Can we have a separate patch with corrected descriptive text?
> 
> Thanks to all involved people.

Here it is :

[PATCH net-next v3] af_unix: Fix use-after-free crashes

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

We are allowed to steal the references to pid/cred only in the last skb
sent from unix_stream_sendmsg(), because first skbs might be consumed by
the receiver before we finish our sendmsg() call.

Remove scm_release() helper, since its cleaner to clear pid/cred fields
in unix_scm_to_skb() when we steal them.

Based on prior patches from Yan Zheng and Tim Chen

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: Jiri Slaby <jirislaby@gmail.com>
Tested-by: Sedat Dilek <sedat.dilek@googlemail.com>
Tested-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
---
 include/net/scm.h  |    9 ---------
 net/unix/af_unix.c |   32 +++++++++++++++++---------------
 2 files changed, 17 insertions(+), 24 deletions(-)

diff --git a/include/net/scm.h b/include/net/scm.h
index 68e1e48..2a5b42f 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -78,15 +78,6 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
 		__scm_destroy(scm);
 }
 
-static __inline__ void scm_release(struct scm_cookie *scm)
-{
-	/* keep ref on pid and cred */
-	scm->pid = NULL;
-	scm->cred = NULL;
-	if (scm->fp)
-		__scm_destroy(scm);
-}
-
 static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
 			       struct scm_cookie *scm)
 {
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index e6d9d10..c8a08ba 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1379,15 +1379,18 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
 }
 
 static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb,
-			   bool send_fds, bool ref)
+			   bool send_fds, bool steal_refs)
 {
 	int err = 0;
-	if (ref) {
+
+	if (!steal_refs) {
 		UNIXCB(skb).pid  = get_pid(scm->pid);
 		UNIXCB(skb).cred = get_cred(scm->cred);
 	} else {
 		UNIXCB(skb).pid  = scm->pid;
 		UNIXCB(skb).cred = scm->cred;
+		scm->pid = NULL;
+		scm->cred = NULL;
 	}
 	UNIXCB(skb).fp = NULL;
 	if (scm->fp && send_fds)
@@ -1454,7 +1457,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	if (skb == NULL)
 		goto out;
 
-	err = unix_scm_to_skb(siocb->scm, skb, true, false);
+	err = unix_scm_to_skb(siocb->scm, skb, true, true);
 	if (err < 0)
 		goto out_free;
 	max_level = err + 1;
@@ -1550,7 +1553,7 @@ restart:
 	unix_state_unlock(other);
 	other->sk_data_ready(other, len);
 	sock_put(other);
-	scm_release(siocb->scm);
+	scm_destroy(siocb->scm);
 	return len;
 
 out_unlock:
@@ -1577,6 +1580,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 	int sent = 0;
 	struct scm_cookie tmp_scm;
 	bool fds_sent = false;
+	bool steal_refs = false;
 	int max_level;
 
 	if (NULL == siocb->scm)
@@ -1638,11 +1642,14 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		size = min_t(int, size, skb_tailroom(skb));
 
 
-		/* Only send the fds and no ref to pid in the first buffer */
-		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, fds_sent);
+		/* Only send the fds in first buffer
+		 * Last buffer can steal our references to pid/cred
+		 */
+		steal_refs = (sent + size >= len);
+		err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, steal_refs);
 		if (err < 0) {
 			kfree_skb(skb);
-			goto out;
+			goto out_err;
 		}
 		max_level = err + 1;
 		fds_sent = true;
@@ -1650,7 +1657,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		err = memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size);
 		if (err) {
 			kfree_skb(skb);
-			goto out;
+			goto out_err;
 		}
 
 		unix_state_lock(other);
@@ -1667,10 +1674,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
 		sent += size;
 	}
 
-	if (skb)
-		scm_release(siocb->scm);
-	else
-		scm_destroy(siocb->scm);
+	scm_destroy(siocb->scm);
 	siocb->scm = NULL;
 
 	return sent;
@@ -1683,9 +1687,7 @@ pipe_err:
 		send_sig(SIGPIPE, current, 0);
 	err = -EPIPE;
 out_err:
-	if (skb == NULL)
-		scm_destroy(siocb->scm);
-out:
+	scm_destroy(siocb->scm);
 	siocb->scm = NULL;
 	return sent ? : err;
 }