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

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

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

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

The following interleaving can therefore occur:

CPU 0 CPU 1 fibnhcupdatemtu() updateorcreatefnhe() load fnhe spinlockbh(&fnhelock) fnheremoveoldest() unlink fnhe kfreercu(fnhe, rcu) <quiescent state> access fnhe after grace period

KASAN reported:

BUG: KASAN: slab-use-after-free in fibnhcupdatemtu+0x3df/0x410 Read of size 8 at addr ffff888107d49000 by task poc/90 Call Trace: fibnhcupdatemtu+0x3df/0x410 fibsyncmtu+0x7a/0xd0 fibnetdevevent+0x229/0x3f0 netifsetmtuext+0x33a/0x570 devsetmtu+0x88/0x120

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

Walk the table under RCU and acquire fnhelock 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.