[PATCH net] sctp: fix error path in sctp_stream_init

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

syzbot noticed a NULL pointer dereference panic in sctp_stream_free()
which was caused by an incomplete error handling in sctp_stream_init().
By not clearing stream->outcnt, it made a for() in sctp_stream_free()
think that it had elements to free, but not, leading to the panic.

As suggested by Xin Long, this patch also simplifies the error path by
moving it to the only if() that uses it.

See-also: https://www.spinics.net/lists/netdev/msg473756.html
See-also: https://www.spinics.net/lists/netdev/msg465024.html
Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: f952be79cebd ("sctp: introduce struct sctp_stream_out_ext")
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
---
 net/sctp/stream.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index 76ea66be0bbee7d3f018676d52c8b95ba06dbcb1..524dfeb94c41ab1ac735746a8acf93af1c96ae48 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -156,9 +156,9 @@ int sctp_stream_init(struct sctp_stream *stream, __u16 outcnt, __u16 incnt,
 	sctp_stream_outq_migrate(stream, NULL, outcnt);
 	sched->sched_all(stream);
 
-	i = sctp_stream_alloc_out(stream, outcnt, gfp);
-	if (i)
-		return i;
+	ret = sctp_stream_alloc_out(stream, outcnt, gfp);
+	if (ret)
+		goto out;
 
 	stream->outcnt = outcnt;
 	for (i = 0; i < stream->outcnt; i++)
@@ -170,19 +170,17 @@ int sctp_stream_init(struct sctp_stream *stream, __u16 outcnt, __u16 incnt,
 	if (!incnt)
 		goto out;
 
-	i = sctp_stream_alloc_in(stream, incnt, gfp);
-	if (i) {
-		ret = -ENOMEM;
-		goto free;
+	ret = sctp_stream_alloc_in(stream, incnt, gfp);
+	if (ret) {
+		sched->free(stream);
+		kfree(stream->out);
+		stream->out = NULL;
+		stream->outcnt = 0;
+		goto out;
 	}
 
 	stream->incnt = incnt;
-	goto out;
 
-free:
-	sched->free(stream);
-	kfree(stream->out);
-	stream->out = NULL;
 out:
 	return ret;
 }
-- 
2.14.3