KVM: x86: Add XSS support for CET_KERNEL and CET_USER

Add CET_KERNEL and CET_USER to KVM's set of supported XSS bits when IBT
*or* SHSTK is supported.  Like CR4.CET, XFEATURE support for IBT and SHSTK
are bundle together under the CET umbrella, and thus prone to
virtualization holes if KVM or the guest supports only one of IBT or SHSTK,
but hardware supports both.  However, again like CR4.CET, such
virtualization holes are benign from the host's perspective so long as KVM
takes care to always honor the "or" logic.

Require CET_KERNEL and CET_USER to come as a pair, and refuse to support
IBT or SHSTK if one (or both) features is missing, as the (host) kernel
expects them to come as a pair, i.e. may get confused and corrupt state if
only one of CET_KERNEL or CET_USER is supported.

Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: Mathias Krause <minipli@grsecurity.net>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
[sean: split to separate patch, write changelog, add XFEATURE_MASK_CET_ALL]
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-26-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: yang-weijiang

arch/x86/kvm/x86.c

@@ -220,13 +220,14 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
 				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
 				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
 
+#define XFEATURE_MASK_CET_ALL	(XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)
 /*
  * Note, KVM supports exposing PT to the guest, but does not support context
  * switching PT via XSTATE (KVM's PT virtualization relies on perf; swapping
  * PT via guest XSTATE would clobber perf state), i.e. KVM doesn't support
  * IA32_XSS[bit 8] (guests can/must use RDMSR/WRMSR to save/restore PT MSRs).
  */
-#define KVM_SUPPORTED_XSS     0
+#define KVM_SUPPORTED_XSS	(XFEATURE_MASK_CET_ALL)
 
 bool __read_mostly allow_smaller_maxphyaddr = 0;
 EXPORT_SYMBOL_GPL(allow_smaller_maxphyaddr);
@@ -10104,6 +10105,16 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
 		kvm_caps.supported_xss = 0;
 
+	if (!kvm_cpu_cap_has(X86_FEATURE_SHSTK) &&
+	    !kvm_cpu_cap_has(X86_FEATURE_IBT))
+		kvm_caps.supported_xss &= ~XFEATURE_MASK_CET_ALL;
+
+	if ((kvm_caps.supported_xss & XFEATURE_MASK_CET_ALL) != XFEATURE_MASK_CET_ALL) {
+		kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+		kvm_cpu_cap_clear(X86_FEATURE_IBT);
+		kvm_caps.supported_xss &= ~XFEATURE_MASK_CET_ALL;
+	}
+
 	if (kvm_caps.has_tsc_control) {
 		/*
 		 * Make sure the user can only configure tsc_khz values that
@@ -12772,10 +12783,11 @@ static void kvm_xstate_reset(struct kvm_vcpu *vcpu, bool init_event)
 	/*
 	 * On INIT, only select XSTATE components are zeroed, most components
 	 * are unchanged.  Currently, the only components that are zeroed and
-	 * supported by KVM are MPX related.
+	 * supported by KVM are MPX and CET related.
 	 */
 	xfeatures_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
-			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR);
+			 (XFEATURE_MASK_BNDREGS | XFEATURE_MASK_BNDCSR |
+			  XFEATURE_MASK_CET_ALL);
 	if (!xfeatures_mask)
 		return;