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

[Joel Fernandes <joelagnelf@nvidia.com>]

On 3/24/2026 5:35 PM, 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>
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>