linux-kernel - Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on xa

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Message-ID: <b0c3552b-1efd-4c48-8d86-91ee16e7222a@rbox.co>
Date: Tue, 6 Aug 2024 18:59:27 +0200
From: Michal Luczaj <mhal@...x.co>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Will Deacon <will@...nel.org>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
 Alexander Potapenko <glider@...gle.com>, Marc Zyngier <maz@...nel.org>
Subject: Re: [PATCH] KVM: Fix error path in kvm_vm_ioctl_create_vcpu() on
 xa_store() failure

On 8/6/24 00:56, Sean Christopherson wrote:
> [...]
> +	/*
> +	 * xa_store() should never fail, see xa_reserve() above.  Leak the vCPU
> +	 * if the impossible happens, as userspace already has access to the
> +	 * vCPU, i.e. freeing the vCPU before userspace puts its file reference
> +	 * would trigger a use-after-free.
> +	 */
>  	if (KVM_BUG_ON(xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0), kvm)) {
> -		r = -EINVAL;
> -		goto kvm_put_xa_release;
> +		mutex_unlock(&vcpu->mutex);
> +		return -EINVAL;
>  	}
>  
>  	/*
> @@ -4302,6 +4310,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  	 */
>  	smp_wmb();
>  	atomic_inc(&kvm->online_vcpus);
> +	mutex_unlock(&vcpu->mutex);
>  
>  	mutex_unlock(&kvm->lock);
>  	kvm_arch_vcpu_postcreate(vcpu);
> @@ -4309,6 +4318,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, unsigned long id)
>  	return r;
>  
>  kvm_put_xa_release:
> +	mutex_unlock(&vcpu->mutex);
>  	kvm_put_kvm_no_destroy(kvm);
>  	xa_release(&kvm->vcpu_array, vcpu->vcpu_idx);

Since we're handling the impossible, isn't the BUG_ON part missing
mutex_unlock(&kvm->lock)?