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Netfilter changes PACKET_OTHERHOST to PACKET_HOST before invoking the
hooks as, while it's an expected value for a bridge, routing expects
PACKET_HOST. The change is undone later on after hook traversal. This
can be seen with pairs of functions updating skb>pkt_type and then
reverting it to its original value:
For hook NF_INET_PRE_ROUTING:
setup_pre_routing / br_nf_pre_routing_finish
For hook NF_INET_FORWARD:
br_nf_forward_ip / br_nf_forward_finish
But the third case where netfilter does this, for hook
NF_INET_POST_ROUTING, the packet type is changed in br_nf_post_routing
but never reverted. A comment says:
/* We assume any code from br_dev_queue_push_xmit onwards doesn't care
* about the value of skb->pkt_type. */
But when having a tunnel (say vxlan) attached to a bridge we have the
following call trace:
br_nf_pre_routing
br_nf_pre_routing_ipv6
br_nf_pre_routing_finish
br_nf_forward_ip
br_nf_forward_finish
br_nf_post_routing <- pkt_type is updated to PACKET_HOST
br_nf_dev_queue_xmit <- but not reverted to its original value
vxlan_xmit
vxlan_xmit_one
skb_tunnel_check_pmtu <- a check on pkt_type is performed
In this specific case, this creates issues such as when an ICMPv6 PTB
should be sent back. When CONFIG_BRIDGE_NETFILTER is enabled, the PTB
isn't sent (as skb_tunnel_check_pmtu checks if pkt_type is PACKET_HOST
and returns early).
If the comment is right and no one cares about the value of
skb->pkt_type after br_dev_queue_push_xmit (which isn't true), resetting
it to its original value should be safe.
Fixes: 1da177e4c3 ("Linux-2.6.12-rc2")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20201123174902.622102-1-atenart@kernel.org
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 Restructured Text 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.