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After commit22600596b6("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:8d7017fd62("blackhole_netdev: use blackhole_netdev to invalidate dst entries") Fixes:22600596b6("ipv4: give an IPv4 dev to blackhole_netdev") Reported-by: Florian Meister <fmei@sfs.com> Closes: https://lore.kernel.org/netdev/20250210084931.23a5c2e4@hermes.local/ Signed-off-by: Ido Schimmel <idosch@nvidia.com> Reviewed-by: Eric Dumazet <edumazet@google.com> Link: https://patch.msgid.link/20250220072559.782296-1-idosch@nvidia.com Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.