sctp: fix race on sctp_id2asoc

BugLink: https://bugs.launchpad.net/bugs/1836654
[ Upstream commit b336decab22158937975293aea79396525f92bb3 ]

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index cc97e082c19f8bff1b13d705e3d22d41cbc9d2f0..836b119f1d8af403cb4dfe8fa544e9f1e8cbf369 100644 (file)
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -236,11 +236,10 @@ 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;
 }