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[PATCH 4.9 20/31] sctp: not allow transport timeout value less than HZ/5 for hb_timer




4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Xin Long <lucien.xin@xxxxxxxxx>

[ Upstream commit 1d88ba1ebb2763aa86172cd7ca05dedbeccc0d35 ]

syzbot reported a rcu_sched self-detected stall on CPU which is caused
by too small value set on rto_min with SCTP_RTOINFO sockopt. With this
value, hb_timer will get stuck there, as in its timer handler it starts
this timer again with this value, then goes to the timer handler again.

This problem is there since very beginning, and thanks to Eric for the
reproducer shared from a syzbot mail.

This patch fixes it by not allowing sctp_transport_timeout to return a
smaller value than HZ/5 for hb_timer, which is based on TCP's min rto.

Note that it doesn't fix this issue by limiting rto_min, as some users
are still using small rto and no proper value was found for it yet.

Reported-by: syzbot+3dcd59a1f907245f891f@xxxxxxxxxxxxxxxxxxxxxxxxx
Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/sctp/transport.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/sctp/transport.c
+++ b/net/sctp/transport.c
@@ -608,7 +608,7 @@ unsigned long sctp_transport_timeout(str
 	    trans->state != SCTP_PF)
 		timeout += trans->hbinterval;
 
-	return timeout;
+	return max_t(unsigned long, timeout, HZ / 5);
 }
 
 /* Reset transport variables to their initial values */