[PATCH v15 29/41] KVM: SEV: Synchronize MSR_IA32_XSS from the GHCB when it's valid

Sean Christopherson
2025-09-12T23:24:18Z

Synchronize XSS from the GHCB to KVM's internal tracking if the guest
marks XSS as valid on a #VMGEXIT. Like XCR0, KVM needs an up-to-date copy
of XSS in order to compute the required XSTATE size when emulating
CPUID.0xD.0x1 for the guest.

Treat the incoming XSS change as an emulated write, i.e. validatate the
guest-provided value, to avoid letting the guest load garbage into KVM's
tracking. Simply ignore bad values, as either the guest managed to get an
unsupported value into hardware, or the guest is misbehaving and providing
pure garbage. In either case, KVM can't fix the broken guest.

Note, emulating the change as an MSR write also takes care of side effects,
e.g. marking dynamic CPUID bits as dirty.

Suggested-by: John Allen <john.allen@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/svm/sev.c | 3 +++
arch/x86/kvm/svm/svm.h | 1 +
2 files changed, 4 insertions(+)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 0cd77a87dd84..0cd32df7b9b6 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -3306,6 +3306,9 @@ static void sev_es_sync_from_ghcb(struct vcpu_svm *svm)
if (kvm_ghcb_xcr0_is_valid(svm))
__kvm_set_xcr(vcpu, 0, kvm_ghcb_get_xcr0(ghcb));

+ if (kvm_ghcb_xss_is_valid(svm))
+ __kvm_emulate_msr_write(vcpu, MSR_IA32_XSS, kvm_ghcb_get_xss(ghcb));
+
/* Copy the GHCB exit information into the VMCB fields */
exit_code = kvm_ghcb_get_sw_exit_code(ghcb);
control->exit_code = lower_32_bits(exit_code);
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index a42e95883b45..10d764878bcc 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -942,5 +942,6 @@ DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_1)
DEFINE_KVM_GHCB_ACCESSORS(sw_exit_info_2)
DEFINE_KVM_GHCB_ACCESSORS(sw_scratch)
DEFINE_KVM_GHCB_ACCESSORS(xcr0)
+DEFINE_KVM_GHCB_ACCESSORS(xss)

#endif
--
2.51.0.384.g4c02a37b29-goog