CVE-2026-99074ipv4: stop PMTU walk when nexthop group shrinks

Official description

In the Linux kernel, the following vulnerability has been resolved: ipv4: stop PMTU walk when nexthop group shrinks Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled") made __ip_rt_update_pmtu() update every path. For nexthop objects, fib_info_num_path() and fib_info_nhc() independently load nh->nh_grp. RCU protects each group's lifetime but does not make the loads observe the same group. If replacement shrinks the group after the loop accepts an index, fib_info_nhc() returns NULL and update_or_create_fnhe() dereferences it. A deterministic probe only widened the existing window between the real operations. On v7.2, an RTM_NEWNEXTHOP replacement published a one-member group after the PMTU reader accepted index 1 from a two-member group, producing: KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: update_or_create_fnhe+0x45/0x15b0 Stop when the indexed path is absent. Groups are dense, so no later index exists in that snapshot. The same real-writer window completed 114,423 PMTU iterations without an oops with this check. A reproducer is available on request. Replacement requires CAP_NET_ADMIN in the network namespace. Where unprivileged user namespaces are permitted, a local user can obtain it in a new user and network namespace.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/82d83eec893d8ae84f280aec44c60d9fdecc62b5
• https://git.kernel.org/stable/c/a411df69524f91fe3adb8f9544d33e701fc9732c
• https://git.kernel.org/stable/c/7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
• https://git.kernel.org/stable/c/c60d17d6dd61adc761ac6f65a008fefbc54cbf87
• https://git.kernel.org/stable/c/c6d670ac656badfd0e15355e07a13a6bdf3b69fd
• https://git.kernel.org/stable/c/f27483f9975b5065ff67054cc7eb8edac6083488
• https://git.kernel.org/stable/c/6619e01855e65ef35723685a557e3eb106f4ecf5