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CVE-2026-74656

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Last modified

CVE-2026-74656 is a vulnerability of currently unknown severity. 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.

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.

Affected Software

Source: CNA advisory (CVE.org). NVD analysis pending.

VendorProductVersions
LinuxLinux>= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < fd39e711866498ae94fcf9acf6f422a4f045b681; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < e1e602d6b22d5cb1641c4459c487eb18bf569e0a; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < e00f7d2b5f2540a3415a229c982af7a25ff6362e; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < dfe388da13aa784851e5ebbea90afbb099075761; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < 5a28a4b22dde92f9d293b94236314b8d6181dc4a; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < 63996ffc594d128ccec8fc0983f91effd2d3adc4; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < ed503eaad62f20cdd5122d7c3078a648a99c8f16; >= af7d6cce53694a88d6a1bb60c9a239a6a5144459, < bc5bde9ce3cc36502839dfe98e068f7303a50982; b427832009b97a3ee412ef643a80b15372a7754d; 2b7e4c735933be79882aba2bed9afa789e03c62f; 8d59c3a6376bbc6dd3f7303968a719ffba75a4f1; 9b4869cf385aa16f89c0f019eed4ec4e36aa441c; ff34695ced21e2dfa04d0fa1c5f6c35011fa8117; >= 3.16.62, < 3.17; >= 4.4.162, < 4.5; >= 4.9.134, < 4.10; >= 4.14.77, < 4.15; >= 4.18.15, < 4.19
LinuxLinux4.19

References

Timeline

Published
Last Modified
Status
Received

Frequently Asked Questions

What is 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.
How severe is CVE-2026-74656?
Severity scoring for CVE-2026-74656 is pending analysis.
How do I fix CVE-2026-74656?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

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Source: NVD / NIST