[PATCH net-next] net/iucv: fix UAF in afiucv_netdev_event()

afiucv_netdev_event() traverses iucv_sk_list without holding
iucv_sk_list.lock.

A concurrent socket teardown can unlink and free an af_iucv socket via
iucv_sock_kill() while the netdevice notifier path is still traversing
the list, resulting in a use-after-free when dereferencing the socket.

syzbot reported a KASAN slab-use-after-free triggered from the netdevice
notifier path:

  BUG: KASAN: slab-use-after-free in afiucv_netdev_event+0x166/0x570
  Read of size 8 at addr 001a54009dcda368 by task syz.3.24/516

  Call Trace:
    afiucv_netdev_event+0x166/0x570 net/iucv/af_iucv.c:2193
    notifier_call_chain+0x18e/0x510 kernel/notifier.c:85
    call_netdevice_notifiers net/core/dev.c:2301 [inline]
    __dev_close_many+0x1c6/0x680 net/core/dev.c:1747
    unregister_netdevice_many_notify+0x9b6/0x2650 net/core/dev.c:12388
    unregister_netdevice_queue+0x392/0x3d0 net/core/dev.c:12291
    __tun_detach+0xa50/0x1990 drivers/net/tun.c:621

  Allocated by task 521:
    sk_prot_alloc+0xd2/0x230 net/core/sock.c:2247
    sk_alloc+0x48/0x680 net/core/sock.c:2303
    iucv_sock_alloc+0x4e/0x710 net/iucv/af_iucv.c:456
    iucv_sock_create+0x114/0x180 net/iucv/af_iucv.c:2254

  Freed by task 519:
    kfree+0x164/0x500 mm/slub.c:6561
    iucv_sock_release+0x12e/0x150 net/iucv/af_iucv.c:1482
    sock_close+0xa4/0x230 net/socket.c:1514

Protect the list traversal with the existing read-side lock.
Use read_lock_bh()/read_unlock_bh() to synchronize with
write_lock_bh()-protected updates to iucv_sk_list.

Fixes: 9fbd87d41392 ("af_iucv: handle netdev events")
Cc: [email protected]
Suggested-by: Hidayath Khan <[email protected]>
Signed-off-by: Nagamani PV <[email protected]>

---
 net/iucv/af_iucv.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/iucv/af_iucv.c b/net/iucv/af_iucv.c
index ea047bab65e7..034039c50941 100644
--- a/net/iucv/af_iucv.c
+++ b/net/iucv/af_iucv.c
@@ -2213,6 +2213,7 @@ static int afiucv_netdev_event(struct notifier_block *this,
 	switch (event) {
 	case NETDEV_REBOOT:
 	case NETDEV_GOING_DOWN:
+		read_lock_bh(&iucv_sk_list.lock);
 		sk_for_each(sk, &iucv_sk_list.head) {
 			iucv = iucv_sk(sk);
 			if ((iucv->hs_dev == event_dev) &&
@@ -2223,6 +2224,7 @@ static int afiucv_netdev_event(struct notifier_block *this,
 				sk->sk_state_change(sk);
 			}
 		}
+		read_unlock_bh(&iucv_sk_list.lock);
 		break;
 	case NETDEV_DOWN:
 	case NETDEV_UNREGISTER:
-- 
2.53.0