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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 20 Feb 2025 07:26:52.1471 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: d592cd57-0f8f-4681-2fe9-08dd517ff237 X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[216.228.117.160];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: BY1PEPF0001AE1C.namprd04.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CY8PR12MB7434 X-Patchwork-Delegate: kuba@kernel.org After commit 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev") IPv4 neighbors can be constructed on the blackhole net device, but they are constructed with an output function (neigh_direct_output()) that simply calls dev_queue_xmit(). The latter will transmit packets via 'skb->dev' which might not be the blackhole net device if dst_dev_put() switched 'dst->dev' to the blackhole net device while another CPU was using the dst entry in ip_output(), but after it already initialized 'skb->dev' from 'dst->dev'. Specifically, the following can happen: CPU1 CPU2 udp_sendmsg(sk1) udp_sendmsg(sk2) udp_send_skb() [...] ip_output() skb->dev = skb_dst(skb)->dev dst_dev_put() dst->dev = blackhole_netdev ip_finish_output2() resolves neigh on dst->dev neigh_output() neigh_direct_output() dev_queue_xmit() This will result in IPv4 packets being sent without an Ethernet header via a valid net device: tcpdump: verbose output suppressed, use -v[v]... for full protocol decode listening on enp9s0, link-type EN10MB (Ethernet), snapshot length 262144 bytes 22:07:02.329668 20:00:40:11:18:fb > 45:00:00:44:f4:94, ethertype Unknown (0x58c6), length 68: 0x0000: 8dda 74ca f1ae ca6c ca6c 0098 969c 0400 ..t....l.l...... 0x0010: 0000 4730 3f18 6800 0000 0000 0000 9971 ..G0?.h........q 0x0020: c4c9 9055 a157 0a70 9ead bf83 38ca ab38 ...U.W.p....8..8 0x0030: 8add ab96 e052 .....R Fix by making sure that neighbors are constructed on top of the blackhole net device with an output function that simply consumes the packets, in a similar fashion to dst_discard_out() and blackhole_netdev_xmit(). Fixes: 8d7017fd621d ("blackhole_netdev: use blackhole_netdev to invalidate dst entries") Fixes: 22600596b675 ("ipv4: give an IPv4 dev to blackhole_netdev") Reported-by: Florian Meister Closes: https://lore.kernel.org/netdev/20250210084931.23a5c2e4@hermes.local/ Signed-off-by: Ido Schimmel Reviewed-by: Eric Dumazet --- drivers/net/loopback.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c index c8840c3b9a1b..f1d68153987e 100644 --- a/drivers/net/loopback.c +++ b/drivers/net/loopback.c @@ -244,8 +244,22 @@ static netdev_tx_t blackhole_netdev_xmit(struct sk_buff *skb, return NETDEV_TX_OK; } +static int blackhole_neigh_output(struct neighbour *n, struct sk_buff *skb) +{ + kfree_skb(skb); + return 0; +} + +static int blackhole_neigh_construct(struct net_device *dev, + struct neighbour *n) +{ + n->output = blackhole_neigh_output; + return 0; +} + static const struct net_device_ops blackhole_netdev_ops = { .ndo_start_xmit = blackhole_netdev_xmit, + .ndo_neigh_construct = blackhole_neigh_construct, }; /* This is a dst-dummy device used specifically for invalidated