CVE-2026-74656
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.
CSIRTS triage
- What
- Use-after-free vulnerability in fib_nhc_update_mtu() IPv4 routing function.
- Who is affected
- Systems running Linux kernel with IPv4 routing enabled.
- Urgency
- Medium severity (CVSS 4.1) use-after-free; not currently exploited but could lead to kernel crashes or privilege escalation.
- Action
- Update to patched Linux kernel version containing the fib_nhc_update_mtu fix.
AI-assisted analysis generated from the source advisory — verify against the original.
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Exploitation outlook
- Low exploitation risk0.18% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 7% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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