Re: [PATCH net v2 1/1] xfrm: use hlist_del_init_rcu for state_cache and state_cache_input

["Siwei Zhang" <oss@fourdim.xyz>]

On Mon, Aug 17, 2026, at 4:30 AM, Steffen Klassert wrote:
> On Thu, Jul 30, 2026 at 07:40:08PM +0800, Siwei Zhang wrote:
>> From: Siwei Zhang <fourdizhang@tencent.com>
>> 
>> Commit 14acf9652e56 ("xfrm: defensively unhash xfrm_state lists in
>> __xfrm_state_delete") converted bydst/bysrc/byseq/byspi from
>> hlist_del_rcu() to hlist_del_init_rcu() so that a second
>> __xfrm_state_delete() on the same object becomes a no-op rather than a
>> write through LIST_POISON pprev. It missed state_cache and
>> state_cache_input, which kept hlist_del_rcu():
>> 
>> - hlist_del_rcu() leaves pprev = LIST_POISON2 (non-NULL), so
>>   hlist_unhashed() returns false.
>> - hlist_del_init_rcu() leaves pprev = NULL, so hlist_unhashed()
>>   returns true.
>> 
>> A second __xfrm_state_delete() therefore enters __hlist_del() on the
>> already-deleted state_cache/state_cache_input nodes and does
>> WRITE_ONCE(*pprev, next) through LIST_POISON2 — a write use-after-free
>> once the slab is reused. The corruption can in turn cause a subsequent
>> hlist_for_each_entry_rcu traversal to follow a dangling next pointer,
>> producing the read use-after-free reported in xfrm_input_state_lookup().
>> 
>> Switch state_cache and state_cache_input to hlist_del_init_rcu() to
>> match the other four lists, closing the write use-after-free and, with
>> it, the read use-after-free it spawns.
>> 
>> Assisted-by: CodeBuddy:GLM-5.2
>> Fixes: 0045e3d80613 ("xfrm: Cache used outbound xfrm states at the policy.")
>> Fixes: 81a331a0e72d ("xfrm: Add an inbound percpu state cache.")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Siwei Zhang <fourdizhang@tencent.com>
>> ---
>>  net/xfrm/xfrm_state.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
>> index 36a4f6793ede..f494c1ac57a4 100644
>> --- a/net/xfrm/xfrm_state.c
>> +++ b/net/xfrm/xfrm_state.c
>> @@ -823,9 +823,9 @@ int __xfrm_state_delete(struct xfrm_state *x)
>>  		if (!hlist_unhashed(&x->byseq))
>>  			hlist_del_init_rcu(&x->byseq);
>>  		if (!hlist_unhashed(&x->state_cache))
>> -			hlist_del_rcu(&x->state_cache);
>> +			hlist_del_init_rcu(&x->state_cache);
>>  		if (!hlist_unhashed(&x->state_cache_input))
>> -			hlist_del_rcu(&x->state_cache_input);
>> +			hlist_del_init_rcu(&x->state_cache_input);
>>  
>>  		if (!hlist_unhashed(&x->byspi))
>>  			hlist_del_init_rcu(&x->byspi);
>
> What is the difference between v1 and v2 of your patch?
> Both look identical to me.

I forgot the net subject prefix in the email subject.

Best,
Siwei