[PATCH 1/2] packet: hold bind lock when rebinding to fanout hook

[Paolo Pisati]

From: Willem de Bruijn <willemb at google.com>

Packet socket bind operations must hold the po->bind_lock. This keeps
po->running consistent with whether the socket is actually on a ptype
list to receive packets.

fanout_add unbinds a socket and its packet_rcv/tpacket_rcv call, then
binds the fanout object to receive through packet_rcv_fanout.

Make it hold the po->bind_lock when testing po->running and rebinding.
Else, it can race with other rebind operations, such as that in
packet_set_ring from packet_rcv to tpacket_rcv. Concurrent updates
can result in a socket being added to a fanout group twice, causing
use-after-free KASAN bug reports, among others.

Reported independently by both trinity and syzkaller.
Verified that the syzkaller reproducer passes after this patch.

Fixes: dc99f600698d ("packet: Add fanout support.")
Reported-by: nixioaming <nixiaoming at huawei.com>
Signed-off-by: Willem de Bruijn <willemb at google.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 008ba2a13f2d04c947adc536d19debb8fe66f110)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 net/packet/af_packet.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 59a1558..2a07eb2 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1317,10 +1317,6 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 
 	mutex_lock(&fanout_mutex);
 
-	err = -EINVAL;
-	if (!po->running)
-		goto out;
-
 	err = -EALREADY;
 	if (po->fanout)
 		goto out;
@@ -1358,7 +1354,10 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 		list_add(&match->list, &fanout_list);
 	}
 	err = -EINVAL;
-	if (match->type == type &&
+
+	spin_lock(&po->bind_lock);
+	if (po->running &&
+	    match->type == type &&
 	    match->prot_hook.type == po->prot_hook.type &&
 	    match->prot_hook.dev == po->prot_hook.dev) {
 		err = -ENOSPC;
@@ -1370,6 +1369,13 @@ static int fanout_add(struct sock *sk, u16 id, u16 type_flags)
 			err = 0;
 		}
 	}
+	spin_unlock(&po->bind_lock);
+
+	if (err && !atomic_read(&match->sk_ref)) {
+		list_del(&match->list);
+		kfree(match);
+	}
+
 out:
 	mutex_unlock(&fanout_mutex);
 	return err;
-- 
2.7.4