CVE-2026-46116

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

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/14acf9652e5690de3c7486c6db5fb8dafd0a32a3
• https://git.kernel.org/stable/c/26edb0a3c99f9d958c212be68b21f1221614dcf0
• https://git.kernel.org/stable/c/2c617848ae6e4f07a3e397f604208c293bbecacc
• https://git.kernel.org/stable/c/3943fcad7694a7d0b15aeabe7d3cc2a2eb8e92e8
• https://git.kernel.org/stable/c/4980162de555cb838f1a189ce7d2cbf5d2e7b050
• https://git.kernel.org/stable/c/a2e2d08fb070fab4947447171f1c4e3ca5a188e5
• https://git.kernel.org/stable/c/b4a53add2fa8f1b5aa17d4c5686c320785fab182