Patch "sctp: use the right sk after waking up from wait_buf sleep" has been added to the 4.4-stable tree

Patch "sctp: use the right sk after waking up from wait_buf sleep" has been added to the 4.4-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

    sctp: use the right sk after waking up from wait_buf sleep

to the 4.4-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     sctp-use-the-right-sk-after-waking-up-from-wait_buf-sleep.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From foo@baz Tue Dec 12 13:38:50 CET 2017
From: Xin Long <lucien.xin@gmail.com>
Date: Wed, 15 Nov 2017 16:57:26 +0800
Subject: sctp: use the right sk after waking up from wait_buf sleep

From: Xin Long <lucien.xin@gmail.com>


[ Upstream commit cea0cc80a6777beb6eb643d4ad53690e1ad1d4ff ]

Commit dfcb9f4f99f1 ("sctp: deny peeloff operation on asocs with threads
sleeping on it") fixed the race between peeloff and wait sndbuf by
checking waitqueue_active(&asoc->wait) in sctp_do_peeloff().

But it actually doesn't work, as even if waitqueue_active returns false
the waiting sndbuf thread may still not yet hold sk lock. After asoc is
peeled off, sk is not asoc->base.sk any more, then to hold the old sk
lock couldn't make assoc safe to access.

This patch is to fix this by changing to hold the new sk lock if sk is
not asoc->base.sk, meanwhile, also set the sk in sctp_sendmsg with the
new sk.

With this fix, there is no more race between peeloff and waitbuf, the
check 'waitqueue_active' in sctp_do_peeloff can be removed.

Thanks Marcelo and Neil for making this clear.

v1->v2:
  fix it by changing to lock the new sock instead of adding a flag in asoc.

Suggested-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 net/sctp/socket.c |   21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -82,8 +82,8 @@
 /* Forward declarations for internal helper functions. */
 static int sctp_writeable(struct sock *sk);
 static void sctp_wfree(struct sk_buff *skb);
-static int sctp_wait_for_sndbuf(struct sctp_association *, long *timeo_p,
-				size_t msg_len);
+static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
+				size_t msg_len, struct sock **orig_sk);
 static int sctp_wait_for_packet(struct sock *sk, int *err, long *timeo_p);
 static int sctp_wait_for_connect(struct sctp_association *, long *timeo_p);
 static int sctp_wait_for_accept(struct sock *sk, long timeo);
@@ -1953,7 +1953,8 @@ static int sctp_sendmsg(struct sock *sk,
 
 	timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
 	if (!sctp_wspace(asoc)) {
-		err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
+		/* sk can be changed by peel off when waiting for buf. */
+		err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len, &sk);
 		if (err) {
 			if (err == -ESRCH) {
 				/* asoc is already dead. */
@@ -4466,12 +4467,6 @@ int sctp_do_peeloff(struct sock *sk, sct
 	if (!asoc)
 		return -EINVAL;
 
-	/* If there is a thread waiting on more sndbuf space for
-	 * sending on this asoc, it cannot be peeled.
-	 */
-	if (waitqueue_active(&asoc->wait))
-		return -EBUSY;
-
 	/* An association cannot be branched off from an already peeled-off
 	 * socket, nor is this supported for tcp style sockets.
 	 */
@@ -6981,7 +6976,7 @@ void sctp_sock_rfree(struct sk_buff *skb
 
 /* Helper function to wait for space in the sndbuf.  */
 static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
-				size_t msg_len)
+				size_t msg_len, struct sock **orig_sk)
 {
 	struct sock *sk = asoc->base.sk;
 	int err = 0;
@@ -7015,11 +7010,17 @@ static int sctp_wait_for_sndbuf(struct s
 		release_sock(sk);
 		current_timeo = schedule_timeout(current_timeo);
 		lock_sock(sk);
+		if (sk != asoc->base.sk) {
+			release_sock(sk);
+			sk = asoc->base.sk;
+			lock_sock(sk);
+		}
 
 		*timeo_p = current_timeo;
 	}
 
 out:
+	*orig_sk = sk;
 	finish_wait(&asoc->wait, &wait);
 
 	/* Release the association's refcnt.  */


Patches currently in stable-queue which might be from lucien.xin@gmail.com are

queue-4.4/route-update-fnhe_expires-for-redirect-when-the-fnhe-exists.patch
queue-4.4/route-also-update-fnhe_genid-when-updating-a-route-cache.patch
queue-4.4/sctp-use-the-right-sk-after-waking-up-from-wait_buf-sleep.patch
queue-4.4/sctp-do-not-free-asoc-when-it-is-already-dead-in-sctp_sendmsg.patch