Re: [PATCH net v2 2/4] ipv4: udp: Create exceptions before socket matching

On Fri, Aug 28, 2026 at 10:23:42PM +0300, Ido Schimmel wrote:

Currently, when ICMP Fragmentation Needed and Redirect Message packets
are locally delivered and quote a UDP packet, a FIB nexthop exception
(FNHE) is only created if the kernel can match the UDP packet to an
existing socket.

This behavior allows off-path attackers to conduct a side-channel attack
on the FNHE cache in order to discover the ephemeral port used by a
connected UDP socket.

Commit 6457378fe796 ("ipv4: use siphash instead of Jenkins in
fnhe_hashfun()") and commit 67d6d681e15b ("ipv4: make exception cache
less predictible") tried to mitigate such attacks by making it harder
for attackers to discover hash collisions in the FNHE cache and by
randomizing the number of exceptions a hash bucket can hold,
respectively. Unfortunately, both of the mitigations can be bypassed.

Instead, mitigate such attacks by always creating a FNHE, even before
trying to find a matching socket. Do that by calling ipv4_update_pmtu()
and ipv4_redirect(), the helpers used when the quoted packet did not
originate from a socket.

This means that guesses (right or wrong) from an off-path attacker will
always result in a FNHE being created or updated in the cache that the
attacker can observe.

Pass an oif of 0, in a similar fashion to icmp_err(). This is also the
oif used by the socket path for sockets that are not bound to a device.

Note that this does not allow attackers to create FNHEs that they could
not create before, as both helpers can already be reached with little to
no validation. For example, by sending an ICMP error that quotes an ICMP
Echo Reply or one that quotes a UDP source port that matches a wildcard
socket.

Also note that in the good case (matched socket) the above scheme comes
at the cost of an extra route lookup, as the no socket helpers perform
their own lookup before the one performed by ipv4_sk_update_pmtu() /
ipv4_sk_redirect(). When the two resolve to different nexthops, it also
results in two exceptions being created for the same destination IP. One
in the FNHE cache of the nexthop resolved by the no socket helpers and
another in the FNHE cache of the nexthop used by the socket.

tl;dr - I don't think anything needs to change here following the
Sashiko feedback [1].

"Does the hard-coded oif of 0 leave the oracle intact for device-bound,
VRF-bound and policy-routed sockets?"

No. An attacker needs to be able to populate the exception cache of the
nexthop used by the socket. If this cache is not reachable via the
no-socket helpers (e.g., because the socket is VRF bound), then the side
channel doesn't exist.

"This isn't a bug, but the new call runs before any socket is known, so the
per-socket opt-out that ipv4_sk_update_pmtu() honours is bypassed:

[...]

For a socket that set IP_PMTUDISC_INTERFACE or IP_PMTUDISC_OMIT, an ICMP
Frag Needed quoting its traffic previously created no PMTU state; now an
exception is installed on the nexthop resolved from the quoted packet.

The socket itself is unaffected, since ip_skb_dst_mtu() returns
min(dst_dev(dst)->mtu, IP_MAX_MTU) when ip_sk_use_pmtu(sk) is false, and
the same socket-less creation is already remotely reachable through
icmp_err() -> ipv4_update_pmtu(skb, net, info, 0, IPPROTO_ICMP). The
selftest patch in this series also asserts this as the intended behaviour
("a socket which disabled PMTU discovery using IP{,V6}_PMTUDISC_OMIT gets
the same exception as the other cases").

Could a sentence be added to the changelog noting that the per-socket
IP_PMTUDISC_INTERFACE / IP_PMTUDISC_OMIT opt-out no longer suppresses
exception creation?"

I don't see a reason to mention this. Exceptions are per-destination
constructs and they could have been created for destinations used by
IP_PMTUDISC_OMIT sockets even before this patch.

[1] https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260828192344.2596928-1-idosch%40nvidia.com