This is a note to let you know that I've just added the patch titled
net: loopback: Avoid sending IP packets without an Ethernet header
to the 6.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
net-loopback-avoid-sending-ip-packets-without-an-eth.patch
and it can be found in the queue-6.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit ec4f8ab14ccc843b21181b75e9f9b0f454be3f8a
Author: Ido Schimmel <idosch@xxxxxxxxxx>
Date: Thu Feb 20 09:25:59 2025 +0200
net: loopback: Avoid sending IP packets without an Ethernet header
[ Upstream commit 0e4427f8f587c4b603475468bb3aee9418574893 ]
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 <fmei@xxxxxxx>
Closes: https://lore.kernel.org/netdev/20250210084931.23a5c2e4@hermes.local/
Signed-off-by: Ido Schimmel <idosch@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Link: https://patch.msgid.link/20250220072559.782296-1-idosch@xxxxxxxxxx
Signed-off-by: Jakub Kicinski <kuba@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/drivers/net/loopback.c b/drivers/net/loopback.c
index 2e9742952c4e9..b213397672d22 100644
--- a/drivers/net/loopback.c
+++ b/drivers/net/loopback.c
@@ -246,8 +246,22 @@ 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
