xfrm: defensively unhash xfrm_state lists in __xfrm_state_delete

Summary

In the Linux kernel, the following vulnerability has been resolved: xfrm: defensively unhash xfrm_state lists in __xfrm_state_delete KASAN reproduces a slab-use-after-free in __xfrm_state_delete()'s hlist_del_rcu calls under syzkaller load on linux-6.12.y stable (reproduced on 6.12.47, also reachable via the same code path on torvalds/master and on the ipsec tree). Nine unique signatures cluster in the xfrm_state lifecycle, the load-bearing one being: BUG: KASAN: slab-use-after-free in __hlist_del include/linux/list.h:990 [inline] BUG: KASAN: slab-use-after-free in hlist_del_rcu include/linux/rculist.h:516 [inline] BUG: KASAN: slab-use-after-free in __xfrm_state_delete net/xfrm/xfrm_state.c Write of size 8 at addr ffff8881198bcb70 by task kworker/u8:9/435 Workqueue: netns cleanup_net Call Trace: __hlist_del / hlist_del_rcu __xfrm_state_delete xfrm_state_delete xfrm_state_flush xfrm_state_fini ops_exit_list cleanup_net The other observed signatures hit the same slab object from __xfrm_state_lookup, xfrm_alloc_spi, __xfrm_state_insert and an OOB write variant of __xfrm_state_delete, all on the byseq/byspi hash chains. __xfrm_state_delete() guards its byseq and byspi unhashes with value-based predicates: if (x->km.seq) hlist_del_rcu(&x->byseq); if (x->id.spi) hlist_del_rcu(&x->byspi); while everywhere else in the file (e.g. state_cache, state_cache_input) the safer hlist_unhashed() check is used. xfrm_alloc_spi() sets x->id.spi = newspi inside xfrm_state_lock and then immediately inserts into byspi, but a path that observes x->id.spi != 0 outside of xfrm_state_lock can still skip-or-hit the byspi unhash inconsistently with whether x is actually on the list. The same holds for x->km.seq versus byseq, and the bydst/bysrc unhashes have no predicate at all, so a second __xfrm_state_delete() on the same object writes through LIST_POISON pprev. The defensive change here: • Use hlist_del_init_rcu() instead of hlist_del_rcu() on bydst, bysrc, byseq and byspi so a second deletion is a no-op rather than a write through LIST_POISON pprev. The byseq/byspi nodes are already initialised in xfrm_state_alloc(). • Test hlist_unhashed() rather than the value predicate for byseq/byspi, so the unhash decision tracks list state rather than mutable scalar fields. Empirical verification: applied this patch on top of v6.12.47, rebuilt, and re-ran the same syzkaller harness for 1h16m on a previously-crashy configuration that produced ~100 hits each of slab-use-after-free Read in xfrm_alloc_spi / Read in __xfrm_state_lookup / Write in __xfrm_state_delete. After the patch, 7.1M execs across 32 VMs at ~1550 exec/sec produced zero xfrm_state UAF/OOB hits. /proc/slabinfo confirms the xfrm_state slab is actively allocated and freed during the run (~143 KiB resident), so the fuzzer is still exercising those code paths -- they just no longer crash. Reproduction: • Linux 6.12.47 x86_64 + KASAN_GENERIC + KASAN_INLINE + KCOV • syzkaller @ 746545b8b1e4c3a128db8652b340d3df90ce61db • 32 QEMU/KVM VMs x 2 vCPU on AWS c5.metal bare metal • 9 unique signatures collected in ~9h, all within xfrm_state lifecycle

Timeline

1. May 28, 2026, 9:35 AM UTCCVE published
2. Oct 16, 2026, 3:19 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Oct 19, 2026, 7:26 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 6b4dc3181b4bfc5f5fc33ab33b1dc6e15759f4b6; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 3943fcad7694a7d0b15aeabe7d3cc2a2eb8e92e8; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 2c617848ae6e4f07a3e397f604208c293bbecacc; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in b4a53add2fa8f1b5aa17d4c5686c320785fab182; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 26edb0a3c99f9d958c212be68b21f1221614dcf0; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 4980162de555cb838f1a189ce7d2cbf5d2e7b050; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in a2e2d08fb070fab4947447171f1c4e3ca5a188e5; From 7b4dc3600e4877178ba94c7fbf7e520421378aa6 · fixed in 14acf9652e5690de3c7486c6db5fb8dafd0a32a3

• Linux Linux

  2.6.19

• KernelLinux

  From 2.6.19 · fixed in 5.10.261; From 5.11.0 · fixed in 5.15.210; From 5.16.0 · fixed in 6.1.176; From 6.2.0 · fixed in 6.6.140; From 6.7.0 · fixed in 6.12.88; From 6.13.0 · fixed in 6.18.30; From 6.19.0 · fixed in 7.0.7

References 8