ipv4: stop PMTU walk when nexthop group shrinks

Overview

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

• Linux >= 3bed2abab4a682d426624ce9add9d7b6d475d4e4 < 7fd60d3c9a1e1b11ef5f1643bbae78d91eae2f4a
• Linux >= 7f5005457b3072fe1ddf15bfef869f2a1d98e05e < c60d17d6dd61adc761ac6f65a008fefbc54cbf87
• Linux >= 7d3f3b4367f315a61fc615e3138f3d320da8c466 < c6d670ac656badfd0e15355e07a13a6bdf3b69fd
• Linux >= 7d3f3b4367f315a61fc615e3138f3d320da8c466 < f27483f9975b5065ff67054cc7eb8edac6083488
• Linux >= 7d3f3b4367f315a61fc615e3138f3d320da8c466 < 6619e01855e65ef35723685a557e3eb106f4ecf5
• Linux fc07a232326a623d0be482e307c08ea7049c2846
• Linux b1f7bdee5750d6cbb9497140cd6582abc5fdcb9e
• Linux >= 6.6.79 < 6.6.160
• Linux >= 6.12.16 < 6.12.113
• Linux >= 5.15.179 < 5.16
• Linux >= 6.1.129 < 6.2
• Linux 6.13

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.