On Wed, Aug 03, 2022, Maxim Levitsky wrote:
> When the guest CPUID doesn't have support for long mode, 32 bit SMRAM
> layout is used and it has no support for preserving EFER and/or SVM
> state.
>
> Note that this isn't relevant to running 32 bit guests on VM which is
> long mode capable - such VM can still run 32 bit guests in compatibility
> mode.
>
> Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> ---
> arch/x86/kvm/svm/svm.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 7ca5e06878e19a..64cfd26bc5e7a6 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4442,6 +4442,15 @@ static int svm_enter_smm(struct kvm_vcpu *vcpu, union kvm_smram *smram)
> if (!is_guest_mode(vcpu))
> return 0;
>
> + /*
> + * 32 bit SMRAM format doesn't preserve EFER and SVM state.
> + * SVM should not be enabled by the userspace without marking
> + * the CPU as at least long mode capable.
Hmm, or userspace can ensure SMIs never get delivered. Maybe?
/*
* 32-bit SMRAM format doesn't preserve EFER and SVM state. Userspace is
* responsible for ensuring nested SVM and SMIs are mutually exclusive.
*/
> + */
> +
Unnecessary newline.
> + if (!guest_cpuid_has(vcpu, X86_FEATURE_LM))
> + return 1;
This doesn't actually fix anything, RSM will still jump to L2 state but in L1
context. I think we first need to actually handle errors from
static_call(kvm_x86_enter_smm).
Given that SVM can't even guarantee nested_svm_simple_vmexit() succeeds, i.e. KVM
can't force the vCPU out of L2 to ensure triple fault would hit L1, killing the VM
seems like the least awful solution (and it's still quite awful).
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 54fa0aa95785..38a6f4089296 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9985,7 +9985,10 @@ static void enter_smm(struct kvm_vcpu *vcpu)
* state (e.g. leave guest mode) after we've saved the state into the
* SMM state-save area.
*/
- static_call(kvm_x86_enter_smm)(vcpu, &smram);
+ if (static_call(kvm_x86_enter_smm)(vcpu, &smram)) {
+ kvm_vm_dead(vcpu->vm);
+ return;
+ }
kvm_smm_changed(vcpu, true);
kvm_vcpu_write_guest(vcpu, vcpu->arch.smbase + 0xfe00, &smram, sizeof(smram));
> +
> smram->smram64.svm_guest_flag = 1;
> smram->smram64.svm_guest_vmcb_gpa = svm->nested.vmcb12_gpa;
>
> --
> 2.26.3
>
