x86/kvm: Avoid freeing stack-allocated node in kvm_async_pf_queue_task

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 <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kernel/kvm.c

···
 	struct swait_queue_head wq;
 	u32 token;
 	int cpu;
+	bool dummy;
 };
 
 static struct kvm_task_sleep_head {
···
 	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);
···
 		}
 		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;