Re: [PATCH v2] x86/kvm: Avoid freeing stack-allocated node in kvm_async_pf_queue_task

Vitaly Kuznetsov
2025-12-10T14:07:40Z

Ryosuke Yasuoka <ryasuoka@redhat.com> writes:

> kvm_async_pf_queue_task() can incorrectly try to kfree() a node
> allocated on the stack of kvm_async_pf_task_wait_schedule().
>
> This occurs when a task requests a PF while another task's PF request
> with the same token is still pending. Since the token is derived from
> the (u32)address in exc_page_fault(), two different tasks can generate
> the same token.
>
> Currently, kvm_async_pf_queue_task() assumes that any entry found in the
> list is a dummy entry and tries to kfree() it. To fix this, add a flag
> to the node structure to distinguish stack-allocated nodes, and only
> kfree() the node if it is a dummy entry.
>
> Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
> ---
>
> v2:
> Based on Vitaly's comment,
> * Update comment in kvm_async_pf_queue_task
> * Set n->dummy false in kvm_async_pf_queue_task
> * Add explanation about what token is in commit message.
>
> v1:
> https://lore.kernel.org/all/87cy4vlmv8.fsf@redhat.com/
>
>
> arch/x86/kernel/kvm.c | 19 ++++++++++++++++---
> 1 file changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index df78ddee0abb..37dc8465e0f5 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -89,6 +89,7 @@ struct kvm_task_sleep_node {
> struct swait_queue_head wq;
> u32 token;
> int cpu;
> + bool dummy;
> };
>
> static struct kvm_task_sleep_head {
> @@ -120,15 +121,26 @@ static bool kvm_async_pf_queue_task(u32 token, struct kvm_task_sleep_node *n)
> raw_spin_lock(&b->lock);
> e = _find_apf_task(b, token);
> if (e) {
> - /* dummy entry exist -> wake up was delivered ahead of PF */
> - hlist_del(&e->link);
> + struct kvm_task_sleep_node *dummy = NULL;
> +
> + /*
> + * The entry can either be a 'dummy' entry (which is put on the
> + * list when wake-up happens ahead of APF handling completion)
> + * or a token from another task which should not be touched.
> + */
> + if (e->dummy) {
> + hlist_del(&e->link);
> + dummy = e;
> + }
> +
> raw_spin_unlock(&b->lock);
> - kfree(e);
> + kfree(dummy);
> return false;
> }
>
> n->token = token;
> n->cpu = smp_processor_id();
> + n->dummy = false;
> init_swait_queue_head(&n->wq);
> hlist_add_head(&n->link, &b->list);
> raw_spin_unlock(&b->lock);
> @@ -231,6 +243,7 @@ static void kvm_async_pf_task_wake(u32 token)
> }
> dummy->token = token;
> dummy->cpu = smp_processor_id();
> + dummy->dummy = true;
> init_swait_queue_head(&dummy->wq);
> hlist_add_head(&dummy->link, &b->list);
> dummy = NULL;
>
> base-commit: 416f99c3b16f582a3fc6d64a1f77f39d94b76de5

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

--
Vitaly