On Wed, Oct 12, 2022, John Allen wrote: > When a guest issues a cpuid instruction for Fn0000000D_x0B > (CetUserOffset), KVM will intercept and need to access the guest s/KVM will/the hypervisor may > XSS value. Heh, "need" is debatable. > For SEV-ES, this is encrypted and needs to be > included in the GHCB to be visible to the hypervisor. The rdmsr > instruction needs to be called directly as the code may be used in early > boot in which case the rdmsr wrappers should be avoided as they are > incompatible with the decompression boot phase. > > Signed-off-by: John Allen <john.allen@xxxxxxx> > --- > This patch is logically part of the SVM guest shadow stack support series seen > here: > https://lore.kernel.org/all/20221012203910.204793-1-john.allen@xxxxxxx/ > > Sending this patch separately from the main series as it should apply to the > tip tree as opposed to the kvm tree as this patch is related to guest kernel > support. > --- > arch/x86/kernel/sev-shared.c | 15 +++++++++++++++ > 1 file changed, 15 insertions(+) > > diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c > index 3a5b0c9c4fcc..34469fac03f0 100644 > --- a/arch/x86/kernel/sev-shared.c > +++ b/arch/x86/kernel/sev-shared.c > @@ -887,6 +887,21 @@ static enum es_result vc_handle_cpuid(struct ghcb *ghcb, > /* xgetbv will cause #GP - use reset value for xcr0 */ > ghcb_set_xcr0(ghcb, 1); > > + if (has_cpuflag(X86_FEATURE_SHSTK) && regs->ax == 0xd) { IIRC, XCR0 and XSS are only needed for sub-leafs 0 and 1, i.e. this and the code above don't need to expose XCR0/XSS to the host for ECX > 1. FWIW, I think it's ridiculous that the guest willingly exposes state to the host, it's not _that_ difficult to do the math in the guest. > + unsigned long lo, hi; > + u64 xss; > + > + /* > + * Since vc_handle_cpuid may be used during early boot, the > + * rdmsr wrappers are incompatible and should not be used. > + * Invoke the instruction directly. > + */ > + asm volatile("rdmsr" : "=a" (lo), "=d" (hi) > + : "c" (MSR_IA32_XSS)); Doesn't __rdmsr() do what you want? But even that seems unnecessary, isn't the current XSS available in xfeatures_mask_supervisor()? > + xss = (hi << 32) | lo; > + ghcb_set_xss(ghcb, xss); > + } > + > ret = sev_es_ghcb_hv_call(ghcb, ctxt, SVM_EXIT_CPUID, 0, 0); > if (ret != ES_OK) > return ret; > -- > 2.34.3 >
