Re: [PATCH] kvm: dirty-ring: Fix race with vcpu creation

[Peter Xu]

Ping

On Tue, Sep 27, 2022 at 11:46:53AM -0400, Peter Xu wrote:
> It's possible that we want to reap a dirty ring on a vcpu that is during
> creation, because the vcpu is put onto list (CPU_FOREACH visible) before
> initialization of the structures.  In this case:
> 
> qemu_init_vcpu
>     x86_cpu_realizefn
>         cpu_exec_realizefn
>             cpu_list_add      <---- can be probed by CPU_FOREACH
>         qemu_init_vcpu
>             cpus_accel->create_vcpu_thread(cpu);
>                 kvm_init_vcpu
>                     map kvm_dirty_gfns  <--- kvm_dirty_gfns valid
> 
> Don't try to reap dirty ring on vcpus during creation or it'll crash.
> 
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2124756
> Reported-by: Xiaohui Li <xiaohli@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  accel/kvm/kvm-all.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> index 5acab1767f..df5fabd3a8 100644
> --- a/accel/kvm/kvm-all.c
> +++ b/accel/kvm/kvm-all.c
> @@ -757,6 +757,15 @@ static uint32_t kvm_dirty_ring_reap_one(KVMState *s, 
> CPUState *cpu)
>      uint32_t ring_size = s->kvm_dirty_ring_size;
>      uint32_t count = 0, fetch = cpu->kvm_fetch_index;
>  
> +    /*
> +     * It's possible that we race with vcpu creation code where the vcpu is
> +     * put onto the vcpus list but not yet initialized the dirty ring
> +     * structures.  If so, skip it.
> +     */
> +    if (!cpu->created) {
> +        return 0;
> +    }
> +
>      assert(dirty_gfns && ring_size);
>      trace_kvm_dirty_ring_reap_vcpu(cpu->cpu_index);
>  
> -- 
> 2.37.3
> 

-- 
Peter Xu