Re: [PATCH] KVM: SEV: Acquire vcpu mutex when updating VMSA

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 14, 2021, Peter Gonda wrote:
> Adds mutex guard to the VMSA updating code. Also adds a check to skip a
> vCPU if it has already been LAUNCH_UPDATE_VMSA'd which should allow
> userspace to retry this ioctl until all the vCPUs can be successfully
> LAUNCH_UPDATE_VMSA'd. Because this operation cannot be undone we cannot
> unwind if one vCPU fails.
> 
> Fixes: ad73109ae7ec ("KVM: SVM: Provide support to launch and run an SEV-ES guest")
> 
> Signed-off-by: Peter Gonda <pgonda@google.com>
> Cc: Marc Orr <marcorr@google.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <seanjc@google.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: kvm@vger.kernel.org
> Cc: stable@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> ---
>  arch/x86/kvm/svm/sev.c | 24 +++++++++++++++++++-----
>  1 file changed, 19 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 75e0b21ad07c..9a2ebd0328ca 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -598,22 +598,29 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>  static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
>  {
>  	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> -	struct sev_data_launch_update_vmsa vmsa;
> +	struct sev_data_launch_update_vmsa vmsa = {0};
>  	struct kvm_vcpu *vcpu;
>  	int i, ret;
>  
>  	if (!sev_es_guest(kvm))
>  		return -ENOTTY;
>  
> -	vmsa.reserved = 0;
> -

Zeroing all of 'vmsa' is an unrelated chagne and belongs in a separate patch.  I
would even go so far as to say it's unnecessary, even field of the struct is
explicitly written before it's consumed.

>  	kvm_for_each_vcpu(i, vcpu, kvm) {
>  		struct vcpu_svm *svm = to_svm(vcpu);
>  
> +		ret = mutex_lock_killable(&vcpu->mutex);
> +		if (ret)
> +			goto out_unlock;

Rather than multiple unlock labels, move the guts of the loop to a wrapper.
As discussed off list, this really should be a vCPU-scoped ioctl, but that ship
has sadly sailed :-(  We can at least imitate that by making the VM-scoped ioctl
nothing but a wrapper.

> +
> +		/* Skip to the next vCPU if this one has already be updated. */

s/be/been

Uber nit, there may not be a next vCPU.  It'd be more slightly more accurate to
say something like "Do nothing if this vCPU has already been updated".

> +		ret = sev_es_sync_vmsa(svm);
> +		if (svm->vcpu.arch.guest_state_protected)
> +			goto unlock;

This belongs in a separate patch, too.  It also introduces a bug (arguably two)
in that it adds a duplicate call to sev_es_sync_vmsa().  The second bug is that
if sev_es_sync_vmsa() fails _and_ the vCPU is already protected, this will cause
that failure to be squashed.

In the end, I think the least gross implementation will look something like this,
implemented over two patches (one for the lock, one for the protected check).

static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
				    int *error)
{
	struct sev_data_launch_update_vmsa vmsa;
	struct vcpu_svm *svm = to_svm(vcpu);
	int ret;

	/*
	 * Do nothing if this vCPU has already been updated.  This is allowed
	 * to let userspace retry LAUNCH_UPDATE_VMSA if the command fails on a
	 * later vCPU.
	 */
	if (svm->vcpu.arch.guest_state_protected)
		return 0;

	/* Perform some pre-encryption checks against the VMSA */
	ret = sev_es_sync_vmsa(svm);
	if (ret)
		return ret;

	/*
	 * The LAUNCH_UPDATE_VMSA command will perform in-place
	 * encryption of the VMSA memory content (i.e it will write
	 * the same memory region with the guest's key), so invalidate
	 * it first.
	 */
	clflush_cache_range(svm->vmsa, PAGE_SIZE);

	vmsa.reserved = 0;
	vmsa.handle = to_kvm_svm(kvm)->sev_info.handle;
	vmsa.address = __sme_pa(svm->vmsa);
	vmsa.len = PAGE_SIZE;
	return sev_issue_cmd(kvm, SEV_CMD_LAUNCH_UPDATE_VMSA, &vmsa, error);
}

static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
{
	struct kvm_vcpu *vcpu;
	int i, ret;

	if (!sev_es_guest(kvm))
		return -ENOTTY;

	kvm_for_each_vcpu(i, vcpu, kvm) {
		ret = mutex_lock_killable(&vcpu->mutex);
		if (ret)
			return ret;

		ret = __sev_launch_update_vmsa(kvm, vcpu, &argp->error);

		mutex_unlock(&vcpu->mutex);
		if (ret)
			return ret;
	}
	return 0;
}