[PATCH net] packet: hold bind lock when rebinding to fanout hook

[Willem de Bruijn]

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.

Reported-by: nixioaming <nixiaom...@huawei.com>
Signed-off-by: Willem de Bruijn <will...@google.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 c26172995511..d288f52c53f7 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1684,10 +1684,6 @@ 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;
@@ -1749,7 +1745,10 @@ 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;
@@ -1761,6 +1760,13 @@ static int fanout_add(struct sock *sk, u16 id, u16 
type_flags)
                        err = 0;
                }
        }
+       spin_unlock(&po->bind_lock);
+
+       if (err && !refcount_read(&match->sk_ref)) {
+               list_del(&match->list);
+               kfree(match);
+       }
+
 out:
        if (err && rollover) {
                kfree(rollover);
-- 
2.14.1.690.gbb1197296e-goog