Re: [PATCH] KVM: SEV-ES: Fix svm_get_msr()/svm_set_msr() for KVM_SEV_ES_INIT guests

[Ravi Bangoria]

On 6/5/2024 5:05 AM, Michael Roth wrote:
> With commit 27bd5fdc24c0 ("KVM: SEV-ES: Prevent MSR access post VMSA
> encryption"), older VMMs like QEMU 9.0 and older will fail when booting
> SEV-ES guests with something like the following error:
> 
>   qemu-system-x86_64: error: failed to get MSR 0x174
>   qemu-system-x86_64: ../qemu.git/target/i386/kvm/kvm.c:3950: kvm_get_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.
> 
> This is because older VMMs that might still call
> svm_get_msr()/svm_set_msr() for SEV-ES guests after guest boot even if
> those interfaces were essentially just noops because of the vCPU state
> being encrypted and stored separately in the VMSA. Now those VMMs will
> get an -EINVAL and generally crash.
> 
> Newer VMMs that are aware of KVM_SEV_INIT2 however are already aware of
> the stricter limitations of what vCPU state can be sync'd during
> guest run-time, so newer QEMU for instance will work both for legacy
> KVM_SEV_ES_INIT interface as well as KVM_SEV_INIT2.
> 
> So when using KVM_SEV_INIT2 it's okay to assume userspace can deal with
> -EINVAL, whereas for legacy KVM_SEV_ES_INIT the kernel might be dealing
> with either an older VMM and so it needs to assume that returning
> -EINVAL might break the VMM.
> 
> Address this by only returning -EINVAL if the guest was started with
> KVM_SEV_INIT2. Otherwise, just silently return.
> 
> Cc: Ravi Bangoria <ravi.bangoria@xxxxxxx>
> Cc: Nikunj A Dadhania <nikunj@xxxxxxx>
> Reported-by: Srikanth Aithal <sraithal@xxxxxxx>
> Closes: https://lore.kernel.org/lkml/37usuu4yu4ok7be2hqexhmcyopluuiqj3k266z4gajc2rcj4yo@eujb23qc3zcm/
> Fixes: 27bd5fdc24c0 ("KVM: SEV-ES: Prevent MSR access post VMSA encryption")
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>

Thanks for the fix.

Reviewed-by: Ravi Bangoria <ravi.bangoria@xxxxxxx>