[PATCH net v2] net: tun: fix race condition between tun_attach and tun_get_user with XDP

[xietangxin <xietangxin@h-partners.com>]

Concurrent TUNSETIFF ioctls and write() operations on the same
file descriptor can trigger a WARN_ONCE in netif_get_rxqueue():

  virt_tun0 received packet on queue 2, but number of RX queues is 2
  WARNING: CPU: 2 PID: 5989 at net/core/dev.c:5494
  Call trace:
   bpf_prog_run_generic_xdp
   netif_receive_generic_xdp
   do_xdp_generic
   tun_get_user
   tun_chr_write_iter

Because tfile->tun is published before tun_set_real_num_queues()
updates dev->real_num_rx_queues. Concurrent tun_get_user() can
observe the newly attached tun file and process packets with
the new queue_index before dev->real_num_rx_queues is updated.

CPU 0 (tun_attach)         CPU 1 (tun_get_user)
tfile->queue_index = 2
rcu_assign_pointer(tfile->tun)
                           skb_record_rx_queue(skb,tfile->queue_index)
                             do_xdp_generic()
                               netif_receive_generic_xdp()
                               bpf_prog_run_generic_xdp()
                               netif_get_rxqueue()
                               // index(2) >= real_num_rx_queues(2)
                                 WARN_ONCE
tun_set_real_num_queues()
  dev->real_num_rx_queues = 3

Fix this by moving rcu_assign_pointer(tfile->tun, tun) after
tun_set_real_num_queues(). This ensures dev->real_num_rx_queues
is updated before any concurrent packet processing path can observe
the attached tun file.

Fixes: 3fe260e00cd0 ("net: tun: record RX queue in skb before do_xdp_generic()")
Cc: stable@vger.kernel.org
Signed-off-by: xietangxin <xietangxin@h-partners.com>
---
v1 -> v2:
- Defer publishing tfile->tun after tun_set_real_num_queues() instead of
  moving numqueues increment, as suggested by Jason Wang, to prevent
  potential TX packet drops.
---
 drivers/net/tun.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 5a302709a68a..2dfca25b86e9 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -811,11 +811,11 @@ static int tun_attach(struct tun_struct *tun, struct file *file,
 	 * initialized tfile; otherwise we risk using half-initialized
 	 * object.
 	 */
-	if (publish_tun)
-		rcu_assign_pointer(tfile->tun, tun);
 	rcu_assign_pointer(tun->tfiles[tun->numqueues], tfile);
 	WRITE_ONCE(tun->numqueues, tun->numqueues + 1);
 	tun_set_real_num_queues(tun);
+	if (publish_tun)
+		rcu_assign_pointer(tfile->tun, tun);
 out:
 	return err;
 }
-- 
2.43.0