On Fri, Jun 23, 2023 at 02:11:46PM -0700, Sean Christopherson wrote: > On Wed, May 24, 2023, John Allen wrote: > > When running as an SEV-ES guest, the PL0_SSP, PL1_SSP, PL2_SSP, PL3_SSP, > > and U_CET fields in the VMCB save area are type B, meaning the host > > state is automatically loaded on a VMEXIT, but is not saved on a VMRUN. > > The other shadow stack MSRs, S_CET, SSP, and ISST_ADDR are type A, > > meaning they are loaded on VMEXIT and saved on VMRUN. Manually save the > > type B host MSR values before VMRUN. > > > > Signed-off-by: John Allen <john.allen@xxxxxxx> > > --- > > arch/x86/kvm/svm/sev.c | 13 +++++++++++++ > > 1 file changed, 13 insertions(+) > > > > diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c > > index c25aeb550cd9..03dd68bddd51 100644 > > --- a/arch/x86/kvm/svm/sev.c > > +++ b/arch/x86/kvm/svm/sev.c > > @@ -3028,6 +3028,19 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa) > > > > /* MSR_IA32_XSS is restored on VMEXIT, save the currnet host value */ > > hostsa->xss = host_xss; > > + > > + if (boot_cpu_has(X86_FEATURE_SHSTK)) { > > + /* > > + * MSR_IA32_U_CET, MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP, > > + * MSR_IA32_PL2_SSP, and MSR_IA32_PL3_SSP are restored on > > + * VMEXIT, save the current host values. > > + */ > > + rdmsrl(MSR_IA32_U_CET, hostsa->u_cet); > > + rdmsrl(MSR_IA32_PL0_SSP, hostsa->vmpl0_ssp); > > + rdmsrl(MSR_IA32_PL1_SSP, hostsa->vmpl1_ssp); > > + rdmsrl(MSR_IA32_PL2_SSP, hostsa->vmpl2_ssp); > > + rdmsrl(MSR_IA32_PL3_SSP, hostsa->vmpl3_ssp); > > Heh, can you send a patch to fix the names for the PLx_SSP fields? They should > be ->plN_ssp, not ->vmplN_ssp. Yes, I will include a patch to address this in the next version of the series. > > As for the values themselves, the kernel doesn't support Supervisor Shadow Stacks > (SSS), so PL0-2_SSP are guaranteed to be zero. And if/when SSS support is added, > I doubt the kernel will ever use PL1_SSP or PL2_SSP, so those can probably be > ignored entirely, and PL0_SSP might be constant per task? In other words, I don't > see any reason to try and track the host values for support that doesn't exist, > just do what VMX does for BNDCFGS and yell if the MSRs are non-zero. Though for > SSS it probably makes sense for KVM to refuse to load (KVM continues on for BNDCFGS > because it's a pretty safe assumption that the kernel won't regain MPX supported). > > E.g. in rough pseudocode > > if (boot_cpu_has(X86_FEATURE_SHSTK)) { > rdmsrl(MSR_IA32_PLx_SSP, host_plx_ssp); > > if (WARN_ON_ONCE(host_pl0_ssp || host_pl1_ssp || host_pl2_ssp)) > return -EIO; > } The function in question returns void and wouldn't be able to return a failure code to callers. We would have to rework this path in order to fail in this way. Is it sufficient to just WARN_ON_ONCE here or is there some other way we can cause KVM to fail to load here?