Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa_store() failure

[Michal Luczaj]

On 7/31/24 15:31, Will Deacon wrote:
> On Tue, Jul 30, 2024 at 04:31:08PM -0700, Sean Christopherson wrote:
>> On Tue, Jul 30, 2024, Michal Luczaj wrote:
>>> On 7/30/24 17:56, Will Deacon wrote:
>>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>>> index d0788d0a72cc..b80dd8cead8c 100644
>>>> --- a/virt/kvm/kvm_main.c
>>>> +++ b/virt/kvm/kvm_main.c
>>>> @@ -4293,7 +4293,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>>>>  
>>>>  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
>>>>  		r = -EINVAL;
>>>> -		goto kvm_put_xa_release;
>>>> +		goto err_xa_release;
>>>>  	}
>>>>  
>>>>  	/*
>>>> @@ -4310,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>>>>  
>>>>  kvm_put_xa_release:
>>>>  	kvm_put_kvm_no_destroy(kvm);
>>>> +err_xa_release:
>>>>  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);
>>>>  unlock_vcpu_destroy:
>>>>  	mutex_unlock(&kvm->lock);
>>>
>>> My bad for neglecting the "impossible" path. Thanks for the fix.
>>>
>>> I wonder if it's complete. If we really want to consider the possibility of
>>> this xa_store() failing, then keeping vCPU fd installed and calling
>>> kmem_cache_free(kvm_vcpu_cache, vcpu) on the error path looks wrong.
>>
>> Yeah, the vCPU is exposed to userspace, freeing its assets will just cause
>> different problems.  KVM_BUG_ON() will prevent _new_ vCPU ioctl() calls (and kick
>> running vCPUs out of the guest), but it doesn't interrupt other CPUs, e.g. if
>> userspace is being sneaking and has already invoked a vCPU ioctl(), KVM will hit
>> a use-after-free (several of them).
> 
> Damn, yes. Just because we haven't returned the fd yet, doesn't mean
> userspace can't make use of it.
>
>> As Michal alluded to, it should be impossible for xa_store() to fail since KVM
>> pre-allocates/reserves memory.  Given that, deliberately leaking the vCPU seems
>> like the least awful "solution".
> 
> Could we actually just move the xa_store() before the fd creation? I
> can't immediately see any issues with that...

Hah, please see commit afb2acb2e3a3 :) Long story short: create_vcpu_fd()
can legally fail, which must be handled gracefully, which would involve
destruction of an already xa_store()ed vCPU, which is racy.