[PATCH net] sctp: check asoc strreset_chunk in sctp_generate_reconf_event

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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 --->
   reconf timer is triggered,
   go to the handler code before hold sk lock
                            <--- reply with Outgoing SSN Reset Request
   process Outgoing SSN Reset Request,
   and set asoc->strreset_chunk to NULL
   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: 7b9438de0cd4 ("sctp: add stream reconf timer")
Reported-by: Ying Xu <yinxu@xxxxxxxxxx>
Signed-off-by: Xin Long <lucien.xin@xxxxxxxxx>
 net/sctp/sm_sideeffect.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/sctp/sm_sideeffect.c b/net/sctp/sm_sideeffect.c
index b3815b568e8e..463c4a58d2c3 100644
--- a/net/sctp/sm_sideeffect.c
+++ b/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,
 			   asoc->state, asoc->ep, asoc,

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