Commit b336deca authored Oct 16, 2018 by Marcelo Ricardo Leitner Committed by David S. Miller Oct 17, 2018

sctp: fix race on sctp_id2asoc



syzbot reported an use-after-free involving sctp_id2asoc.  Dmitry Vyukov
helped to root cause it and it is because of reading the asoc after it
was freed:

        CPU 1                       CPU 2
(working on socket 1)            (working on socket 2)
	                         sctp_association_destroy
sctp_id2asoc
   spin lock
     grab the asoc from idr
   spin unlock
                                   spin lock
				     remove asoc from idr
				   spin unlock
				   free(asoc)
   if asoc->base.sk != sk ... [*]

This can only be hit if trying to fetch asocs from different sockets. As
we have a single IDR for all asocs, in all SCTP sockets, their id is
unique on the system. An application can try to send stuff on an id
that matches on another socket, and the if in [*] will protect from such
usage. But it didn't consider that as that asoc may belong to another
socket, it may be freed in parallel (read: under another socket lock).

We fix it by moving the checks in [*] into the protected region. This
fixes it because the asoc cannot be freed while the lock is held.

Reported-by:  <syzbot+c7dd55d7aec49d48e49a@syzkaller.appspotmail.com>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 9675931e

net/sctp/socket.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -271,11 +271,10 @@ struct sctp_association sctp_id2assoc(struct sock sk, sctp_assoc_t id)

		spin_lock_bh(&sctp_assocs_id_lock);
		asoc = (struct sctp_association *)idr_find(&sctp_assocs_id, (int)id);
		if (asoc && (asoc->base.sk != sk \|\| asoc->base.dead))
		asoc = NULL;
		spin_unlock_bh(&sctp_assocs_id_lock);

		if (!asoc \|\| (asoc->base.sk != sk) \|\| asoc->base.dead)
		return NULL;

		return asoc;
		}

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