[PATCH] net: tun: Fix use-after-free in tun_detach()

[Shigeru Yoshida]

syzbot reported use-after-free in tun_detach() [1].  This causes call
trace like below:

==================================================================
BUG: KASAN: use-after-free in notifier_call_chain+0x1da/0x1e0
...
Call Trace:
 <TASK>
 dump_stack_lvl+0x100/0x178
 print_report+0x167/0x470
 ? __virt_addr_valid+0x5e/0x2d0
 ? __phys_addr+0xc6/0x140
 ? notifier_call_chain+0x1da/0x1e0
 ? notifier_call_chain+0x1da/0x1e0
 kasan_report+0xbf/0x1e0
 ? notifier_call_chain+0x1da/0x1e0
 notifier_call_chain+0x1da/0x1e0
 call_netdevice_notifiers_info+0x83/0x130
 netdev_run_todo+0xc33/0x11b0
 ? generic_xdp_install+0x490/0x490
 ? __tun_detach+0x1500/0x1500
 tun_chr_close+0xe2/0x190
 __fput+0x26a/0xa40
 task_work_run+0x14d/0x240
 ? task_work_cancel+0x30/0x30
 do_exit+0xb31/0x2a40
 ? reacquire_held_locks+0x4a0/0x4a0
 ? do_raw_spin_lock+0x12e/0x2b0
 ? mm_update_next_owner+0x7c0/0x7c0
 ? rwlock_bug.part.0+0x90/0x90
 ? lockdep_hardirqs_on_prepare+0x17f/0x410
 do_group_exit+0xd4/0x2a0
 __x64_sys_exit_group+0x3e/0x50
 do_syscall_64+0x38/0xb0
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The cause of the issue is that sock_put() from __tun_detach() drops
last reference count for struct net, and then notifier_call_chain()
from netdev_state_change() accesses that struct net.

This patch fixes the issue by calling sock_put() from tun_detach()
after all necessary accesses for the struct net has done.

Link: https://syzkaller.appspot.com/bug?id=96eb7f1ce75ef933697f24eeab928c4a716edefe [1]
Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
---
 drivers/net/tun.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 7a3ab3427369..ce9fcf4c8ef4 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -686,7 +686,6 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
 		if (tun)
 			xdp_rxq_info_unreg(&tfile->xdp_rxq);
 		ptr_ring_cleanup(&tfile->tx_ring, tun_ptr_free);
-		sock_put(&tfile->sk);
 	}
 }
 
@@ -702,6 +701,11 @@ static void tun_detach(struct tun_file *tfile, bool clean)
 	if (dev)
 		netdev_state_change(dev);
 	rtnl_unlock();
+
+	if (clean) {
+		synchronize_rcu();
+		sock_put(&tfile->sk);
+	}
 }
 
 static void tun_detach_all(struct net_device *dev)
-- 
2.38.1