sctp: check asoc strreset_chunk in sctp_generate_reconf_event

A null pointer reference issue can be triggered when the response of a
stream reconf request arrives after the timer is triggered, such as:

  send Incoming SSN Reset Request --->
  CPU0:
   reconf timer is triggered,
   go to the handler code before hold sk lock
                            <--- reply with Outgoing SSN Reset Request
  CPU1:
   process Outgoing SSN Reset Request,
   and set asoc->strreset_chunk to NULL
  CPU0:
   continue the handler code, hold sk lock,
   and try to hold asoc->strreset_chunk, crash!

In Ying Xu's testing, the call trace is:

  [ ] BUG: kernel NULL pointer dereference, address: 0000000000000010
  [ ] RIP: 0010:sctp_chunk_hold+0xe/0x40 [sctp]
  [ ] Call Trace:
  [ ]  <IRQ>
  [ ]  sctp_sf_send_reconf+0x2c/0x100 [sctp]
  [ ]  sctp_do_sm+0xa4/0x220 [sctp]
  [ ]  sctp_generate_reconf_event+0xbd/0xe0 [sctp]
  [ ]  call_timer_fn+0x26/0x130

This patch is to fix it by returning from the timer handler if asoc
strreset_chunk is already set to NULL.

Fixes: 7b9438d ("sctp: add stream reconf timer")
Reported-by: Ying Xu <yinxu@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/sm_sideeffect.c

@@ -458,6 +458,10 @@ void sctp_generate_reconf_event(struct timer_list *t)
 		goto out_unlock;
 	}
 
+	/* This happens when the response arrives after the timer is triggered. */
+	if (!asoc->strreset_chunk)
+		goto out_unlock;
+
 	error = sctp_do_sm(net, SCTP_EVENT_T_TIMEOUT,
 			   SCTP_ST_TIMEOUT(SCTP_EVENT_TIMEOUT_RECONF),
 			   asoc->state, asoc->ep, asoc,