sctp: clear out_curr if all frag chunks of current msg are pruned

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2160516
Tested: compile only

commit 2f201ae
Author: Xin Long <lucien.xin@gmail.com>
Date:   Fri Nov 4 17:45:16 2022 -0400

    sctp: clear out_curr if all frag chunks of current msg are pruned

    A crash was reported by Zhen Chen:

      list_del corruption, ffffa035ddf01c18->next is NULL
      WARNING: CPU: 1 PID: 250682 at lib/list_debug.c:49 __list_del_entry_valid+0x59/0xe0
      RIP: 0010:__list_del_entry_valid+0x59/0xe0
      Call Trace:
       sctp_sched_dequeue_common+0x17/0x70 [sctp]
       sctp_sched_fcfs_dequeue+0x37/0x50 [sctp]
       sctp_outq_flush_data+0x85/0x360 [sctp]
       sctp_outq_uncork+0x77/0xa0 [sctp]
       sctp_cmd_interpreter.constprop.0+0x164/0x1450 [sctp]
       sctp_side_effects+0x37/0xe0 [sctp]
       sctp_do_sm+0xd0/0x230 [sctp]
       sctp_primitive_SEND+0x2f/0x40 [sctp]
       sctp_sendmsg_to_asoc+0x3fa/0x5c0 [sctp]
       sctp_sendmsg+0x3d5/0x440 [sctp]
       sock_sendmsg+0x5b/0x70

    and in sctp_sched_fcfs_dequeue() it dequeued a chunk from stream
    out_curr outq while this outq was empty.

    Normally stream->out_curr must be set to NULL once all frag chunks of
    current msg are dequeued, as we can see in sctp_sched_dequeue_done().
    However, in sctp_prsctp_prune_unsent() as it is not a proper dequeue,
    sctp_sched_dequeue_done() is not called to do this.

    This patch is to fix it by simply setting out_curr to NULL when the
    last frag chunk of current msg is dequeued from out_curr stream in
    sctp_prsctp_prune_unsent().

    Fixes: 5bbbbe3 ("sctp: introduce stream scheduler foundations")
    Reported-by: Zhen Chen <chenzhen126@huawei.com>
    Tested-by: Caowangbao <caowangbao@huawei.com>
    Signed-off-by: Xin Long <lucien.xin@gmail.com>
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Xin Long <lxin@redhat.com>

Author: Xin Long

net/sctp/outqueue.c

@@ -403,6 +403,11 @@ static int sctp_prsctp_prune_unsent(struct sctp_association *asoc,
 		sout = SCTP_SO(&asoc->stream, chk->sinfo.sinfo_stream);
 		sout->ext->abandoned_unsent[SCTP_PR_INDEX(PRIO)]++;
 
+		/* clear out_curr if all frag chunks are pruned */
+		if (asoc->stream.out_curr == sout &&
+		    list_is_last(&chk->frag_list, &chk->msg->chunks))
+			asoc->stream.out_curr = NULL;
+
 		msg_len -= chk->skb->truesize + sizeof(struct sctp_chunk);
 		sctp_chunk_free(chk);
 		if (msg_len <= 0)