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

[Ryosuke Yasuoka]

On Wed, Dec 10, 2025 at 03:07:34PM +0100, Vitaly Kuznetsov wrote:
> Ryosuke Yasuoka <ryasuoka@xxxxxxxxxx> 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@xxxxxxxxxx>
> > ---
> >
> > 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@xxxxxxxxxx/
> >
> >
> >  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@xxxxxxxxxx>
> 
> -- 
> Vitaly

Vitaly,

Thank you for your review.

Hi Paolo,

Just a gentle reminder regarding this patch. It has received a
Reviewed-by tag from Vitaly.

Since this patch fixes a bug encountered by our customers, we would
appreciate it if you could consider applying it.

Let me know if there is anything else needed on my end!

Regards,
Ryosuke