sctp: Fix double free in sctp_sendmsg_to_asoc

syzbot/kasan detected a double free in sctp_sendmsg_to_asoc:
BUG: KASAN: use-after-free in sctp_association_free+0x7b7/0x930
net/sctp/associola.c:332
Read of size 8 at addr ffff8801d8006ae0 by task syzkaller914861/4202

CPU: 1 PID: 4202 Comm: syzkaller914861 Not tainted 4.16.0-rc4+ #258
Hardware name: Google Google Compute Engine/Google Compute Engine
01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x24d lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x23c/0x360 mm/kasan/report.c:412
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
 sctp_association_free+0x7b7/0x930 net/sctp/associola.c:332
 sctp_sendmsg+0xc67/0x1a80 net/sctp/socket.c:2075
 inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:763
 sock_sendmsg_nosec net/socket.c:629 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:639
 SYSC_sendto+0x361/0x5c0 net/socket.c:1748
 SyS_sendto+0x40/0x50 net/socket.c:1716
 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

This was introduced by commit:
f84af33 sctp: factor out sctp_sendmsg_to_asoc from sctp_sendmsg

As the newly refactored function moved the wait_for_sndbuf call to a
point after the association was connected, allowing for peeloff events
to occur, which in turn caused wait_for_sndbuf to return -EPIPE which
was not caught by the logic that determines if an association should be
freed or not.

Fix it the easy way by returning the ordering of
sctp_primitive_ASSOCIATE and sctp_wait_for_sndbuf to the old order, to
ensure that EPIPE will not happen.

Tested by myself using the syzbot reproducers with positive results

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: davem@davemloft.net
CC: Xin Long <lucien.xin@gmail.com>
Reported-by: syzbot+a4e4112c3aff00c8cfd8@syzkaller.appspotmail.com
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index aeecdd6..7a10ae3 100644 (file)
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1883,6 +1883,19 @@ static int sctp_sendmsg_to_asoc(struct sctp_association *asoc,
                goto err;
        }
 
+       if (asoc->pmtu_pending)
+               sctp_assoc_pending_pmtu(asoc);
+
+       if (sctp_wspace(asoc) < msg_len)
+               sctp_prsctp_prune(asoc, sinfo, msg_len - sctp_wspace(asoc));
+
+       if (!sctp_wspace(asoc)) {
+               timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
+               err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
+               if (err)
+                       goto err;
+       }
+
        if (sctp_state(asoc, CLOSED)) {
                err = sctp_primitive_ASSOCIATE(net, asoc, NULL);
                if (err)
@@ -1900,19 +1913,6 @@ static int sctp_sendmsg_to_asoc(struct sctp_association *asoc,
                pr_debug("%s: we associated primitively\n", __func__);
        }
 
-       if (asoc->pmtu_pending)
-               sctp_assoc_pending_pmtu(asoc);
-
-       if (sctp_wspace(asoc) < msg_len)
-               sctp_prsctp_prune(asoc, sinfo, msg_len - sctp_wspace(asoc));
-
-       if (!sctp_wspace(asoc)) {
-               timeo = sock_sndtimeo(sk, msg->msg_flags & MSG_DONTWAIT);
-               err = sctp_wait_for_sndbuf(asoc, &timeo, msg_len);
-               if (err)
-                       goto err;
-       }
-
        datamsg = sctp_datamsg_from_user(asoc, sinfo, &msg->msg_iter);
        if (IS_ERR(datamsg)) {
                err = PTR_ERR(datamsg);