UBUNTU-CVE-2026-64572: Use After Free
In the Linux kernel, the following vulnerability has been resolved: ipv4: fib: free fibalias with kfreercu() on insert error path fibtableinsert() publishes newfa into the leaf's falist with fibinsertalias() before calling the fib entry notifiers. When a notifier fails, the error path removes newfa with fibremovealias() (hlistdelrcu) and frees it right away with kmemcachefree(). fibtablelookup() walks that list under rcureadlock() only, so a concurrent lookup that already reached newfa keeps reading it after the free: BUG: KASAN: slab-use-after-free in fibtablelookup (net/ipv4/fibtrie.c:1601) Read of size 1 at addr ffff88810676d4eb by task exploit/297 Call Trace: fibtablelookup (net/ipv4/fibtrie.c:1601) iprouteoutputkeyhashrcu (net/ipv4/route.c:2814) iprouteoutputkeyhash (net/ipv4/route.c:2705) ip4datagramconnect (net/ipv4/datagram.c:49) udpconnect (net/ipv4/udp.c:2144) sysconnect (net/socket.c:2167) x64sysconnect (net/socket.c:2173) dosyscall64 entrySYSCALL64afterhwframe which belongs to the cache ipfibalias of size 56 Triggering the error path needs CAPNETADMIN and a registered fib notifier that can reject a route; a netdevsim device whose IPv4 FIB resource is exhausted is enough. Free newfa with aliasfreememrcu(), as fibtabledelete() already does for a fibalias removed from the trie.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-aws-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1067.70+fips1 - Upgrade
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ubuntu/linux-gcp-fipsto a version that resolves this vulnerability.Fixed in 6.8.0-1071.79+fips1 - Upgrade
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ubuntu/linuxto a version that resolves this vulnerability.Fixed in 7.0.0-38.38 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1014.14 - Upgrade
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ubuntu/linux-azureto a version that resolves this vulnerability.Fixed in 7.0.0-1017.17 - Upgrade
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ubuntu/linux-azure-fdeto a version that resolves this vulnerability.Fixed in 7.0.0-1016.16 - Upgrade
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ubuntu/linux-gcpto a version that resolves this vulnerability.Fixed in 7.0.0-1014.14 - Upgrade
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ubuntu/linux-ibmto a version that resolves this vulnerability.Fixed in 7.0.0-1015.15 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-38.38.1 - Compensating control
In fib_table_insert()'s notifier-error path, free new_fa with alias_free_mem_rcu() (which uses kfree_rcu()) after fib_remove_alias(), instead of freeing it immediately with kmem_cache_free().
Event History
Frequently Asked Questions
Who is realistically exposed to this issue?
Exploitation requires CAP_NET_ADMIN and a registered FIB notifier that can reject a route. Systems where untrusted users or workloads can obtain that capability and influence route configuration are the relevant exposure cases.
What condition is required to trigger the vulnerable error path?
A route insertion must publish a new FIB alias, then a FIB entry notifier must fail and reject the route. A concurrent IPv4 route lookup that has already reached the removed alias can then read the object after it is freed.
How can I determine whether this issue has been triggered?
The provided failure signature is a KASAN slab use-after-free report in fib_table_lookup(), involving an object from the ip_fib_alias cache. The example call path reaches fib_table_lookup() through IPv4 route output during a UDP connect operation.