KVM: x86: Account for KVM-reserved CR4 bits when passing through CR4 …

…on VMX

Drop x86.c's local pre-computed cr4_reserved bits and instead fold KVM's
reserved bits into the guest's reserved bits.  This fixes a bug where VMX's
set_cr4_guest_host_mask() fails to account for KVM-reserved bits when
deciding which bits can be passed through to the guest.  In most cases,
letting the guest directly write reserved CR4 bits is ok, i.e. attempting
to set the bit(s) will still #GP, but not if a feature is available in
hardware but explicitly disabled by the host, e.g. if FSGSBASE support is
disabled via "nofsgsbase".

Note, the extra overhead of computing host reserved bits every time
userspace sets guest CPUID is negligible.  The feature bits that are
queried are packed nicely into a handful of words, and so checking and
setting each reserved bit costs in the neighborhood of ~5 cycles, i.e. the
total cost will be in the noise even if the number of checked CR4 bits
doubles over the next few years.  In other words, x86 will run out of CR4
bits long before the overhead becomes problematic.

Note ChimeraOS#2, __cr4_reserved_bits() starts from CR4_RESERVED_BITS, which is
why the existing __kvm_cpu_cap_has() processing doesn't explicitly OR in
CR4_RESERVED_BITS (and why the new code doesn't do so either).

Fixes: 2ed41aa ("KVM: VMX: Intercept guest reserved CR4 bits to inject #GP fault")
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20241128013424.4096668-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/cpuid.c

@@ -421,8 +421,11 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
 
 	kvm_pmu_refresh(vcpu);
-	vcpu->arch.cr4_guest_rsvd_bits =
-	    __cr4_reserved_bits(guest_cpuid_has, vcpu);
+
+#define __kvm_cpu_cap_has(UNUSED_, f) kvm_cpu_cap_has(f)
+	vcpu->arch.cr4_guest_rsvd_bits = __cr4_reserved_bits(__kvm_cpu_cap_has, UNUSED_) |
+					 __cr4_reserved_bits(guest_cpuid_has, vcpu);
+#undef __kvm_cpu_cap_has
 
 	kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu->arch.cpuid_entries,
 						    vcpu->arch.cpuid_nent));

arch/x86/kvm/x86.c

@@ -119,8 +119,6 @@ u64 __read_mostly efer_reserved_bits = ~((u64)(EFER_SCE | EFER_LME | EFER_LMA));
 static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
 #endif
 
-static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
-
 #define KVM_EXIT_HYPERCALL_VALID_MASK (1 << KVM_HC_MAP_GPA_RANGE)
 
 #define KVM_CAP_PMU_VALID_MASK KVM_PMU_CAP_DISABLE
@@ -1285,9 +1283,6 @@ EXPORT_SYMBOL_GPL(kvm_emulate_xsetbv);
 
 bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
-	if (cr4 & cr4_reserved_bits)
-		return false;
-
 	if (cr4 & vcpu->arch.cr4_guest_rsvd_bits)
 		return false;
 
@@ -9773,10 +9768,6 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
 	if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
 		kvm_caps.supported_xss = 0;
 
-#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_caps.has_tsc_control) {
 		/*
 		 * Make sure the user can only configure tsc_khz values that