CVE-2026-99076: ipv6: udp: Create exceptions before socket matching

In the Linux kernel, the following vulnerability has been resolved:

ipv6: udp: Create exceptions before socket matching

Currently, when ICMPv6 Packet Too Big and Redirect Message packets are locally delivered and quote a UDP packet, an exception is only created in the IPv6 exception cache 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 exception cache in order to discover the ephemeral port used by a connected UDP socket.

Commit 4785305c05b2 ("ipv6: use siphash in rt6exceptionhash()") and commit a00df2caffed ("ipv6: make exception cache less predictible") tried to mitigate such attacks by making it harder for attackers to discover hash collisions in the exception 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 an exception, even before trying to find a matching socket. Do that by calling ip6updatepmtu() and ip6redirect(), 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 an exception being created or updated in the cache that the attacker can observe.

Pass the ifindex of the ingress device and the default uid, in a similar fashion to icmpv6err(). Unlike IPv4, an oif of 0 would not match any nexthop in ip6redirectnhmatch() and no exception would be created in response to a Redirect Message.

Note that this does not allow attackers to create exceptions that they could not create before, as both helpers can already be reached with little to no validation. For example, by sending an ICMPv6 error that quotes an ICMPv6 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 ip6skupdatepmtu() / ip6skredirect(). When the two resolve to different nexthops, it also results in two exceptions being created for the same destination IP. One in the exception cache of the nexthop resolved by the no socket helpers and another in the exception cache of the nexthop used by the socket.