[PATCH net V1] veth: prevent NULL pointer dereference in veth_xdp_rcv

[Jesper Dangaard Brouer <hawk@kernel.org>]

The veth peer device is RCU protected, but when the peer device gets
deleted (veth_dellink) then the pointer is assigned NULL (via
RCU_INIT_POINTER).

This patch adds a necessary NULL check in veth_xdp_rcv when accessing
the veth peer net_device.

This fixes a bug introduced in commit dc82a33297fc ("veth: apply qdisc
backpressure on full ptr_ring to reduce TX drops"). The bug is a race
and only triggers when having inflight packets on a veth that is being
deleted.

Reported-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Closes: https://lore.kernel.org/all/fecfcad0-7a16-42b8-bff2-66ee83a6e5c4@linux.dev/
Reported-by: syzbot+c4c7bf27f6b0c4bd97fe@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/683da55e.a00a0220.d8eae.0052.GAE@google.com/
Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
---
Commit dc82a33297fc have reached Linus'es tree in v6.16-rc1~132^2~208^2.
Thus, we can correct this before the release of v6.16.

 drivers/net/veth.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index e58a0f1b5c5b..a3046142cb8e 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -909,7 +909,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
 
 	/* NAPI functions as RCU section */
 	peer_dev = rcu_dereference_check(priv->peer, rcu_read_lock_bh_held());
-	peer_txq = netdev_get_tx_queue(peer_dev, queue_idx);
+	peer_txq = peer_dev ? netdev_get_tx_queue(peer_dev, queue_idx) : NULL;
 
 	for (i = 0; i < budget; i++) {
 		void *ptr = __ptr_ring_consume(&rq->xdp_ring);
@@ -959,7 +959,7 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget,
 	rq->stats.vs.xdp_packets += done;
 	u64_stats_update_end(&rq->stats.syncp);
 
-	if (unlikely(netif_tx_queue_stopped(peer_txq)))
+	if (peer_txq && unlikely(netif_tx_queue_stopped(peer_txq)))
 		netif_tx_wake_queue(peer_txq);
 
 	return done;