Re: [PATCH] slab,rcu: disable KVFREE_RCU_BATCHED for strict grace period

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 3/25/26 09:21, Harry Yoo (Oracle) wrote:
> On Wed, Mar 25, 2026 at 08:50:07AM +0100, Vlastimil Babka (SUSE) wrote:
>> On 3/24/26 22:35, Jann Horn wrote:
>> > Disable CONFIG_KVFREE_RCU_BATCHED in CONFIG_RCU_STRICT_GRACE_PERIOD builds
>> > so that kernel fuzzers have an easier time finding use-after-free involving
>> > kfree_rcu().
>> > 
>> > The intent behind CONFIG_RCU_STRICT_GRACE_PERIOD is that RCU should invoke
>> > callbacks and free objects as soon as possible (at a large performance
>> > cost) so that kernel fuzzers and such have an easier time detecting
>> > use-after-free bugs in objects with RCU lifetime.
>> > 
>> > CONFIG_KVFREE_RCU_BATCHED is a performance optimization that queues
>> > RCU-freed objects in ways that CONFIG_RCU_STRICT_GRACE_PERIOD can't
>> > expedite; for example, the following testcase doesn't trigger a KASAN splat
>> > when CONFIG_KVFREE_RCU_BATCHED is enabled:
>> > ```
>> > struct foo_struct {
>> >   struct rcu_head rcu;
>> >   int a;
>> > };
>> > struct foo_struct *foo = kmalloc(sizeof(*foo),
>> >     GFP_KERNEL | __GFP_NOFAIL | __GFP_ZERO);
>> > 
>> > pr_info("%s: calling kfree_rcu()\n", __func__);
>> > kfree_rcu(foo, rcu);
>> > msleep(10);
>> > pr_info("%s: start UAF access\n", __func__);
>> > READ_ONCE(foo->a);
>> > pr_info("%s: end UAF access\n", __func__);
>> > ```
>> > 
>> > Signed-off-by: Jann Horn <jannh@google.com>
>> 
>> Hm but with 7.0 we have sheaves everywhere including kmalloc caches, and
>> there's a percpu rcu_free sheaf collecting kfree_rcu'd objects.
> 
> Right, but only when CONFIG_KVFREE_RCU_BATCHED=y
> 
>> Only when
>> it's full it's submitted to call_rcu() where the callback rcu_free_sheaf()
>> runs slab_free_hook() including kasan hooks etc. If there's nothing filling
>> the rcu_free sheaf, the objects can sit there possibly indefinitely.
> 
> Right.
> 
>> That means CONFIG_KVFREE_RCU_BATCHED now handles only the rare cases where
>> kfree_rcu() to the rcu_free sheaf fails (and I still owe it to Ulad to do
>> something about this).
> 
> Right.
> 
>> So to complete the intent of this patch, we should perhaps also skip the
>> rcu_free sheaf with RCU_STRICT_GRACE_PERIOD? (or with !KVFREE_RCU_BATCHED
>> perhaps as it's also a form of batching).
> 
> Maybe I'm missing something, but...
> 
> by making KVFREE_RCU_BATCHED depend on !RCU_STRICT_GRACE_PERIOD,
> selecting RCU_STRICT_GRACE_PERIOD disables all uses of rcu_free sheaves?
> 
> kvfree_call_rcu() implementation on !KVFREE_RCU_BATCHED does not call
> kfree_rcu_sheaf().

Ah yeah, I missed that there are two kvfree_call_rcu() implementations and
kfree_rcu_sheaf() is only used in the batched one. Sorry for the noise.

Will queue the patch

>> But then I wonder if the testcase in the changelog appeared to be fixed with
>> this patch on a 7.0-rcX kernel (base-commit: below is rc3+) because by my
>> understanding it shouldn't have been. (unless there happened to be enough
>> kfree_rcu() activity on that cpu+kmalloc cache combination, so that the
>> rcu_free sheaf got submitted withing that msleep(10)).
>> 
>> > ---
>> >  mm/Kconfig | 1 +
>> >  1 file changed, 1 insertion(+)
>> > 
>> > diff --git a/mm/Kconfig b/mm/Kconfig
>> > index ebd8ea353687..67a72fe89186 100644
>> > --- a/mm/Kconfig
>> > +++ b/mm/Kconfig
>> > @@ -172,6 +172,7 @@ config SLUB
>> >  config KVFREE_RCU_BATCHED
>> >  	def_bool y
>> >  	depends on !SLUB_TINY && !TINY_RCU
>> > +	depends on !RCU_STRICT_GRACE_PERIOD
>> >  
>> >  config SLUB_TINY
>> >  	bool "Configure for minimal memory footprint"
>> > 
>> > ---
>> > base-commit: b29fb8829bff243512bb8c8908fd39406f9fd4c3
>> > change-id: 20260324-kasan-kfree-rcu-4e7f490237ef
>> > 
>> > --  
>> > Jann Horn <jannh@google.com>
>> > 
>> 
>> 
>