CVE-2026-74656: ipv4: fix use-after-free in fib_nhc_update_mtu()

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

ipv4: fix use-after-free in fib_nhc_update_mtu()

fib_nhc_update_mtu() walks the nexthop exception table under RTNL, but
RTNL does not serialize this walk with PMTU exception updates. The walk
uses rcu_dereference_protected() with a constant true condition without
holding fnhe_lock.

The following interleaving can therefore occur:

  CPU 0                              CPU 1
  fib_nhc_update_mtu()               update_or_create_fnhe()
    load fnhe                          spin_lock_bh(&fnhe_lock)
                                       fnhe_remove_oldest()
                                         unlink fnhe
                                         kfree_rcu(fnhe, rcu)
    <quiescent state>
    access fnhe after grace period

KASAN reported:

  BUG: KASAN: slab-use-after-free in fib_nhc_update_mtu+0x3df/0x410
  Read of size 8 at addr ffff888107d49000 by task poc/90
  Call Trace:
   fib_nhc_update_mtu+0x3df/0x410
   fib_sync_mtu+0x7a/0xd0
   fib_netdev_event+0x229/0x3f0
   netif_set_mtu_ext+0x33a/0x570
   dev_set_mtu+0x88/0x120

The same walk updates fnhe_pmtu and fnhe_mtu_locked. These fields form a
pair and other writers serialize them with fnhe_lock. RCU alone prevents
reclamation, but would still allow concurrent writers to leave a mixed
pair.

Walk the table under RCU and acquire fnhe_lock only while updating each
exception. RCU keeps the current entry alive while the short critical
section serializes its paired PMTU fields. This avoids holding the global
lock while scanning all 2048 buckets for every nexthop.

The Linux kernel CVE team has assigned CVE-2026-74656 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 5.10.265 with commit fd39e711866498ae94fcf9acf6f422a4f045b681
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 5.15.216 with commit e1e602d6b22d5cb1641c4459c487eb18bf569e0a
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 6.1.183 with commit e00f7d2b5f2540a3415a229c982af7a25ff6362e
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 6.6.152 with commit dfe388da13aa784851e5ebbea90afbb099075761
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 6.12.104 with commit 5a28a4b22dde92f9d293b94236314b8d6181dc4a
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 6.18.45 with commit 63996ffc594d128ccec8fc0983f91effd2d3adc4
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 7.1.9 with commit ed503eaad62f20cdd5122d7c3078a648a99c8f16
	Issue introduced in 4.19 with commit af7d6cce53694a88d6a1bb60c9a239a6a5144459 and fixed in 7.2 with commit bc5bde9ce3cc36502839dfe98e068f7303a50982
	Issue introduced in 3.16.62 with commit b427832009b97a3ee412ef643a80b15372a7754d
	Issue introduced in 4.4.162 with commit 2b7e4c735933be79882aba2bed9afa789e03c62f
	Issue introduced in 4.9.134 with commit 8d59c3a6376bbc6dd3f7303968a719ffba75a4f1
	Issue introduced in 4.14.77 with commit 9b4869cf385aa16f89c0f019eed4ec4e36aa441c
	Issue introduced in 4.18.15 with commit ff34695ced21e2dfa04d0fa1c5f6c35011fa8117

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74656
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	include/net/route.h
	net/ipv4/fib_semantics.c
	net/ipv4/route.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/fd39e711866498ae94fcf9acf6f422a4f045b681
	https://git.kernel.org/stable/c/e1e602d6b22d5cb1641c4459c487eb18bf569e0a
	https://git.kernel.org/stable/c/e00f7d2b5f2540a3415a229c982af7a25ff6362e
	https://git.kernel.org/stable/c/dfe388da13aa784851e5ebbea90afbb099075761
	https://git.kernel.org/stable/c/5a28a4b22dde92f9d293b94236314b8d6181dc4a
	https://git.kernel.org/stable/c/63996ffc594d128ccec8fc0983f91effd2d3adc4
	https://git.kernel.org/stable/c/ed503eaad62f20cdd5122d7c3078a648a99c8f16
	https://git.kernel.org/stable/c/bc5bde9ce3cc36502839dfe98e068f7303a50982