[PATCH v4 2/6] KVM: x86: Filter out XTILE_CFG if XTILE_DATA isn't permitted

[Sean Christopherson]

Filter out XTILE_CFG from the supported XCR0 reported to userspace if the
current process doesn't have access to XTILE_DATA.  Attempting to set
XTILE_CFG in XCR0 will #GP if XTILE_DATA is also not set, and so keeping
XTILE_CFG as supported results in explosions if userspace feeds
KVM_GET_SUPPORTED_CPUID back into KVM and the guest doesn't sanity check
CPUID.

Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at KVM_GET_SUPPORTED_CPUID")
Reported-by: Aaron Lewis <aaronlewis@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/kvm/x86.h | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index b6c6988d99b5..3402d69820da 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -3,6 +3,7 @@
 #define ARCH_X86_KVM_X86_H
 
 #include <linux/kvm_host.h>
+#include <asm/fpu/xstate.h>
 #include <asm/mce.h>
 #include <asm/pvclock.h>
 #include "kvm_cache_regs.h"
@@ -325,7 +326,22 @@ extern bool enable_pmu;
  */
 static inline u64 kvm_get_filtered_xcr0(void)
 {
-	return kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
+	u64 permitted_xcr0 = kvm_caps.supported_xcr0;
+
+	BUILD_BUG_ON(XFEATURE_MASK_USER_DYNAMIC != XFEATURE_MASK_XTILE_DATA);
+
+	if (permitted_xcr0 & XFEATURE_MASK_USER_DYNAMIC) {
+		permitted_xcr0 &= xstate_get_guest_group_perm();
+
+		/*
+		 * Treat XTILE_CFG as unsupported if the current process isn't
+		 * allowed to use XTILE_DATA, as attempting to set XTILE_CFG in
+		 * XCR0 without setting XTILE_DATA is architecturally illegal.
+		 */
+		if (!(permitted_xcr0 & XFEATURE_MASK_XTILE_DATA))
+			permitted_xcr0 &= ~XFEATURE_MASK_XTILE_CFG;
+	}
+	return permitted_xcr0;
 }
 
 static inline bool kvm_mpx_supported(void)
-- 
2.40.0.348.gf938b09366-goog