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

["Harry Yoo (Oracle)" <harry@kernel.org>]

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().

> 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>
> > 
> 
> 

-- 
Cheers,
Harry / Hyeonggon