[PATCH] kvm: x86: Use KVM CPU capabilities to determine CR4 reserved bits

[Paolo Bonzini]

Using CPUID data can be useful for the processor compatibility
check, but that's it.  Using it to compute guest-reserved bits
can have both false positives (such as LA57 and UMIP which we
are already handling) and false negatives: in particular, with
this patch we don't allow anymore a KVM guest to set CR4.PKE
when CR4.PKE is clear on the host.

Fixes: b9dd21e104bc ("KVM: x86: simplify handling of PKRU")
Reported-by: Jim Mattson <jmattson@xxxxxxxxxx>
Tested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/x86.c | 20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 45688d075044..e0639b2c332e 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -929,19 +929,6 @@ EXPORT_SYMBOL_GPL(kvm_set_xcr);
 	__reserved_bits;				\
 })
 
-static u64 kvm_host_cr4_reserved_bits(struct cpuinfo_x86 *c)
-{
-	u64 reserved_bits = __cr4_reserved_bits(cpu_has, c);
-
-	if (kvm_cpu_cap_has(X86_FEATURE_LA57))
-		reserved_bits &= ~X86_CR4_LA57;
-
-	if (kvm_cpu_cap_has(X86_FEATURE_UMIP))
-		reserved_bits &= ~X86_CR4_UMIP;
-
-	return reserved_bits;
-}
-
 static int kvm_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
 	if (cr4 & cr4_reserved_bits)
@@ -9674,7 +9661,9 @@ int kvm_arch_hardware_setup(void *opaque)
 	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
 		supported_xss = 0;
 
-	cr4_reserved_bits = kvm_host_cr4_reserved_bits(&boot_cpu_data);
+#define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
+	cr4_reserved_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_);
+#undef __kvm_cpu_cap_has
 
 	if (kvm_has_tsc_control) {
 		/*
@@ -9706,7 +9695,8 @@ int kvm_arch_check_processor_compat(void *opaque)
 
 	WARN_ON(!irqs_disabled());
 
-	if (kvm_host_cr4_reserved_bits(c) != cr4_reserved_bits)
+	if (__cr4_reserved_bits(cpu_has, c) !=
+	    __cr4_reserved_bits(cpu_has, &boot_cpu_data))
 		return -EIO;
 
 	return ops->check_processor_compatibility();
-- 
2.18.2