[PATCH net] sctp: fix error handling on stream scheduler initialization

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

It allocates the extended area for outbound streams only on sendmsg
calls, if they are not yet allocated.  When using the priority
stream scheduler, this initialization may imply into a subsequent
allocation, which may fail.  In this case, it was aborting the stream
scheduler initialization but leaving the ->ext pointer (allocated) in
there, thus in a partially initialized state.  On a subsequent call to
sendmsg, it would notice the ->ext pointer in there, and trip on
uninitialized stuff when trying to schedule the data chunk.

The fix is undo the ->ext initialization if the stream scheduler
initialization fails and avoid the partially initialized state.

Although syzkaller bisected this to commit 4ff40b86262b ("sctp: set
chunk transport correctly when it's a new asoc"), this bug was actually
introduced on the commit I marked below.

Reported-by: syzbot+c1a380d42b190ad1e559@syzkaller.appspotmail.com
Fixes: 5bbbbe32a431 ("sctp: introduce stream scheduler foundations")
Tested-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
---
 net/sctp/stream.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index 93ed07877337eace4ef7f4775dda5868359ada37..25946604af85c09917e63e5c4a8d7d6fa2caebc4 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -153,13 +153,20 @@ int sctp_stream_init(struct sctp_stream *stream, __u16 outcnt, __u16 incnt,
 int sctp_stream_init_ext(struct sctp_stream *stream, __u16 sid)
 {
 	struct sctp_stream_out_ext *soute;
+	int ret;
 
 	soute = kzalloc(sizeof(*soute), GFP_KERNEL);
 	if (!soute)
 		return -ENOMEM;
 	SCTP_SO(stream, sid)->ext = soute;
 
-	return sctp_sched_init_sid(stream, sid, GFP_KERNEL);
+	ret = sctp_sched_init_sid(stream, sid, GFP_KERNEL);
+	if (ret) {
+		kfree(SCTP_SO(stream, sid)->ext);
+		SCTP_SO(stream, sid)->ext = NULL;
+	}
+
+	return ret;
 }
 
 void sctp_stream_free(struct sctp_stream *stream)
-- 
2.21.0