Re: [PATCH net] tunnels: Drop stale dst when building an ICMP error for PMTUD

[Aaron Conole <aconole@redhat.com>]

Ido Schimmel <idosch@nvidia.com> writes:

> Bridged UDP tunnels such as VXLAN and GENEVE build an ICMP error packet
> around an overlay packet if the packet is going to exceed the underlay
> path MTU. The ICMP error packet is then injected back into the Rx path
> with the source and destination addresses swapped, so that it will be
> delivered to the overlay source.
>
> If the overlay packet was routed to the UDP tunnel or locally generated,
> then it is already carrying a valid dst entry and this entry is not
> dropped when transforming the packet to an ICMP error packet. This
> causes the IP layer to reuse the dst entry, leading to the ICMP error
> packet being dropped or routed out of the UDP tunnel interface in case
> of forwarding.
>
> Prior to the blamed commit this could not happen, as
> skb_tunnel_check_pmtu() did not build ICMP errors for PACKET_HOST
> packets. Such packets were instead encapsulated and, unless the DF bit
> was set in the outer header, fragmented by the underlay.
>
> Fix this by making sure that the ICMP error packet does not have a valid
> dst entry, thereby forcing the IP layer to perform a route lookup.
>
> Adjust the bridged PMTU exception selftests accordingly. When the
> local sender in ns_a pings the overlay destination with a deadline
> (-w), ping exits on the first socket error before any reply is
> received and returns a non-zero exit code. The test therefore only
> passed because the ICMP error was never delivered. Use a packet count
> (-c) like the ns_c line above it, so that the ICMP error counts
> against the packet budget and the exit code depends on whether echo
> replies were received. This passes with and without the fix.
>
> Fixes: 8930424777e4 ("tunnels: Accept PACKET_HOST in skb_tunnel_check_pmtu().")
> Cc: stable@vger.kernel.org
> Reported-by: Laika Price <laikabcprice@gmail.com>
> Closes: https://lore.kernel.org/netdev/20260614-master-v3-1-9f5060ba1ed1@gmail.com/
> Reported-by: Yaroslav Dudkov <aroslavdudkov622@gmail.com>
> Closes: https://lore.kernel.org/netdev/20260901081825.287173-1-aroslavdudkov622@gmail.com/
> Reported-by: Charles Bordet <rough.rock3059@datachamp.fr>
> Closes: https://lore.kernel.org/netdev/aHVhQLPJIhq-SYPM@eldamar.lan/
> Signed-off-by: Ido Schimmel <idosch@nvidia.com>
> ---

Thanks for the detailed analysis.  As noted, it was a very obscure and
subtle issue.

Reviewed-by: Aaron Conole <aconole@redhat.com>