CVE-2017-6353: Double Free (CWE-415)
value: net/sctp/socket.c in the Linux kernel through 4.10.1 does not properly restrict association peel-off operations during certain wait states, which allows local users to cause a denial of service (invalid unlock and double free) via a multithreaded application.  NOTE: this vulnerability exists because of an incorrect fix for CVE-2017-5986.
CWE description: The product calls free() twice on the same memory address, potentially leading to modification of unexpected memory locations.
CVSS v3 base score: 5.5
Published: 2017-03-01T20:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/sctp/socket.c
Commit date: 2017-02-24 11:10:38 -0500
Commit message: sctp: deny peeloff operation on asocs with threads sleeping on it

commit 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
attempted to avoid a BUG_ON call when the association being used for a
sendmsg() is blocked waiting for more sndbuf and another thread did a
peeloff operation on such asoc, moving it to another socket.

As Ben Hutchings noticed, then in such case it would return without
locking back the socket and would cause two unlocks in a row.

Further analysis also revealed that it could allow a double free if the
application managed to peeloff the asoc that is created during the
sendmsg call, because then sctp_sendmsg() would try to free the asoc
that was created only for that call.

This patch takes another approach. It will deny the peeloff operation
if there is a thread sleeping on the asoc, so this situation doesn't
exist anymore. This avoids the issues described above and also honors
the syscalls that are already being handled (it can be multiple sendmsg
calls).

Joint work with Xin Long.

Fixes: 2dcab5984841 ("sctp: avoid BUG_ON on sctp_wait_for_sndbuf")
Cc: Alexander Popov <alex.popov@linux.com>
Cc: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index b5321486fbed..465a9c8464f9 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -4860,10 +4860,16 @@ int sctp_do_peeloff(struct sock *sk, sctp_assoc_t id, struct socket **sockp)
 	int err = 0;
 
 	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.
 	 */
 	if (!sctp_style(sk, UDP))
 		return -EINVAL;
@@ -7597,12 +7603,10 @@ static int sctp_wait_for_sndbuf(struct sctp_association *asoc, long *timeo_p,
 		/* Let another process have a go.  Since we are going
 		 * to sleep anyway.
 		 */
 		release_sock(sk);
 		current_timeo = schedule_timeout(current_timeo);
-		if (sk != asoc->base.sk)
-			goto do_error;
 		lock_sock(sk);
 
 		*timeo_p = current_timeo;
 	}
 

Vulnerable code: 		if (sk != asoc->base.sk)
			goto do_error;
Fixed code: 	/* 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;