[PATCHv3 net] sctp: check and update stream->out_curr when allocating stream_out

[Xin Long]

Now when using stream reconfig to add out streams, stream->out
will get re-allocated, and all old streams' information will
be copied to the new ones and the old ones will be freed.

So without stream->out_curr updated, next time when trying to
send from stream->out_curr stream, a panic would be caused.

This patch is to check and update stream->out_curr when
allocating stream_out.

v1->v2:
  - define fa_index() to get elem index from stream->out_curr.
v2->v3:
  - repost with no change.

Fixes: 5bbbbe32a431 ("sctp: introduce stream scheduler foundations")
Reported-by: Ying Xu <yi...@redhat.com>
Reported-by: syzbot+e33a3a138267ca119...@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien....@gmail.com>
---
 net/sctp/stream.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/net/sctp/stream.c b/net/sctp/stream.c
index 80e0ae5..f246331 100644
--- a/net/sctp/stream.c
+++ b/net/sctp/stream.c
@@ -84,6 +84,19 @@ static void fa_zero(struct flex_array *fa, size_t index, 
size_t count)
        }
 }
 
+static size_t fa_index(struct flex_array *fa, void *elem, size_t count)
+{
+       size_t index = 0;
+
+       while (count--) {
+               if (elem == flex_array_get(fa, index))
+                       break;
+               index++;
+       }
+
+       return index;
+}
+
 /* Migrates chunks from stream queues to new stream queues if needed,
  * but not across associations. Also, removes those chunks to streams
  * higher than the new max.
@@ -147,6 +160,13 @@ static int sctp_stream_alloc_out(struct sctp_stream 
*stream, __u16 outcnt,
 
        if (stream->out) {
                fa_copy(out, stream->out, 0, min(outcnt, stream->outcnt));
+               if (stream->out_curr) {
+                       size_t index = fa_index(stream->out, stream->out_curr,
+                                               stream->outcnt);
+
+                       BUG_ON(index == stream->outcnt);
+                       stream->out_curr = flex_array_get(out, index);
+               }
                fa_free(stream->out);
        }
 
-- 
2.1.0