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

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 <ryasuoka@redhat.com>
Message-ID: <20251206140939.144038-1-ryasuoka@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>