[PATCH net] sctp: fix the issue that pathmtu may be set lower than MINSEGMENT

[Xin Long <lucien.xin@gmail.com>]

After commit b6c5734db070 ("sctp: fix the handling of ICMP Frag Needed
for too small MTUs"), sctp_transport_update_pmtu would refetch pathmtu
from the dst and set it to transport's pathmtu without any check.

The new pathmtu may be lower than MINSEGMENT if the dst is obsolete and
updated by .get_dst() in sctp_transport_update_pmtu.

Syzbot reported a warning in sctp_mtu_payload caused by this.

This fix uses the refetched pathmtu only when it's greater than the
frag_needed pmtu.

Fixes: b6c5734db070 ("sctp: fix the handling of ICMP Frag Needed for too small MTUs")
Reported-by: syzbot+f0d9d7cba052f9344b03@syzkaller.appspotmail.com
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/sctp/transport.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/sctp/transport.c b/net/sctp/transport.c
index 445b7ef..ddfb687 100644
--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -282,7 +282,10 @@ bool sctp_transport_update_pmtu(struct sctp_transport *t, u32 pmtu)
 
 	if (dst) {
 		/* Re-fetch, as under layers may have a higher minimum size */
-		pmtu = SCTP_TRUNC4(dst_mtu(dst));
+		u32 mtu = SCTP_TRUNC4(dst_mtu(dst));
+
+		if (pmtu < mtu)
+			pmtu = mtu;
 		change = t->pathmtu != pmtu;
 	}
 	t->pathmtu = pmtu;
-- 
2.1.0