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

Peter Gonda
2021-09-14T22:39:56Z

On Tue, Sep 14, 2021 at 3:34 PM Sean Christopherson <seanjc@google.com> wrote:
>
> 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.

I'll remove this.

>
> > 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.

I'll move skipping logic to a seperate patch

>
> 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;
> }

That looks reasonable to me. I didn't know if changes headed for LTS
should be smaller so I avoided doing this refactor. From:
https://www.kernel.org/doc/html/v4.11/process/stable-kernel-rules.html#stable-kernel-rules
seems to say less than 100 lines is ideal. I guess this could also be
a "theoretical race condition” anyways so maybe not for LTS anyways.
Thoughts?