Re: [PATCH V2] KVM: SEV: Acquire vcpu mutex when updating VMSA

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/09/21 00:40, Marc Orr wrote:
> On Wed, Sep 15, 2021 at 10:18 AM Peter Gonda <pgonda@google.com> wrote:
>>
>> Adds vcpu mutex guard to the VMSA updating code. Refactors out
>> __sev_launch_update_vmsa() function to deal with per vCPU parts
>> of sev_launch_update_vmsa().
> 
> Can you expand the changelog, and perhaps add a comment into the
> source code as well, to explain what grabbing the mutex protects us
> from? I assume that it's a poorly behaved user-space, rather than a
> race condition in a well-behaved user-space VMM, but I'm not certain.
> 
> Other than that, the patch itself seems fine to me.

I added this:

The update-VMSA ioctl touches data stored in struct kvm_vcpu, and
therefore should not be performed concurrently with any VCPU ioctl
that might cause KVM or the processor to use the same data.

Paolo

>>
>> 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
>> ---
>>
>> V2
>>   * Refactor per vcpu work to separate function.
>>   * Remove check to skip already initialized VMSAs.
>>   * Removed vmsa struct zeroing.
>>
>> ---
>>   arch/x86/kvm/svm/sev.c | 53 ++++++++++++++++++++++++------------------
>>   1 file changed, 30 insertions(+), 23 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 75e0b21ad07c..766510fe3abb 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -595,43 +595,50 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>>          return 0;
>>   }
>>
>> -static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp)
>> +static int __sev_launch_update_vmsa(struct kvm *kvm, struct kvm_vcpu *vcpu,
>> +                                   int *error)
>>   {
>> -       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
>>          struct sev_data_launch_update_vmsa vmsa;
>> +       struct vcpu_svm *svm = to_svm(vcpu);
>> +       int ret;
>> +
>> +       /* 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;
>>
>> -       vmsa.reserved = 0;
>> -
>> -       kvm_for_each_vcpu(i, vcpu, kvm) {
>> -               struct vcpu_svm *svm = to_svm(vcpu);
>> -
>> -               /* Perform some pre-encryption checks against the VMSA */
>> -               ret = sev_es_sync_vmsa(svm);
>> +       kvm_for_each_vcpu(i, vcpu, kvm) {
>> +               ret = mutex_lock_killable(&vcpu->mutex);
>>                  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);
>> +               ret = __sev_launch_update_vmsa(kvm, vcpu, &argp->error);
>>
>> -               vmsa.handle = sev->handle;
>> -               vmsa.address = __sme_pa(svm->vmsa);
>> -               vmsa.len = PAGE_SIZE;
>> -               ret = sev_issue_cmd(kvm, SEV_CMD_LAUNCH_UPDATE_VMSA, &vmsa,
>> -                                   &argp->error);
>> +               mutex_unlock(&vcpu->mutex);
>>                  if (ret)
>>                          return ret;
>> -
>> -               svm->vcpu.arch.guest_state_protected = true;
>>          }
>>
>>          return 0;
>> --
>> 2.33.0.464.g1972c5931b-goog
>>
>