[59/74] unix: fix a race condition in unix_release()

[Ben Hutchings]

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

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

From: Paul Moore <pmoore@xxxxxxxxxx>

[ Upstream commit ded34e0fe8fe8c2d595bfa30626654e4b87621e0 ]

As reported by Jan, and others over the past few years, there is a
race condition caused by unix_release setting the sock->sk pointer
to NULL before properly marking the socket as dead/orphaned.  This
can cause a problem with the LSM hook security_unix_may_send() if
there is another socket attempting to write to this partially
released socket in between when sock->sk is set to NULL and it is
marked as dead/orphaned.  This patch fixes this by only setting
sock->sk to NULL after the socket has been marked as dead; I also
take the opportunity to make unix_release_sock() a void function
as it only ever returned 0/success.

Dave, I think this one should go on the -stable pile.

Special thanks to Jan for coming up with a reproducer for this
problem.

Reported-by: Jan Stancek <jan.stancek@xxxxxxxxx>
Signed-off-by: Paul Moore <pmoore@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 net/unix/af_unix.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 317bfe3..18978b6 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -371,7 +371,7 @@ static void unix_sock_destructor(struct sock *sk)
 #endif
 }
 
-static int unix_release_sock(struct sock *sk, int embrion)
+static void unix_release_sock(struct sock *sk, int embrion)
 {
 	struct unix_sock *u = unix_sk(sk);
 	struct dentry *dentry;
@@ -444,8 +444,6 @@ static int unix_release_sock(struct sock *sk, int embrion)
 
 	if (unix_tot_inflight)
 		unix_gc();		/* Garbage collect fds */
-
-	return 0;
 }
 
 static void init_peercred(struct sock *sk)
@@ -682,9 +680,10 @@ static int unix_release(struct socket *sock)
 	if (!sk)
 		return 0;
 
+	unix_release_sock(sk, 0);
 	sock->sk = NULL;
 
-	return unix_release_sock(sk, 0);
+	return 0;
 }
 
 static int unix_autobind(struct socket *sock)

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