[v3,21/57] KVM: x86: Account for max supported CPUID leaf when getting raw host CPUID

Commit Message

Sean Christopherson Nov. 28, 2024, 1:33 a.m. UTC

Explicitly zero out the feature word in kvm_cpu_caps if the word's
associated CPUID function is greater than the max leaf supported by the
CPU.  For such unsupported functions, Intel CPUs return the output from
the last supported leaf, not all zeros.

Practically speaking, this is likely a benign bug, as KVM uses the raw
host CPUID to mask the kernel's computed capabilities, and the kernel does
perform max leaf checks when populating boot_cpu_data.  The only way KVM's
goof could be problematic is if the kernel force-set a feature in a leaf
that is completely unsupported, _and_ the max supported leaf happened to
return a value with '1' the same bit position.  Which is theoretically
possible, but extremely unlikely.  And even if that did happen, it's
entirely possible that KVM would still provide the correct functionality;
the kernel did set the capability after all.

Reviewed-by: Maxim Levitsky <[e-mail]>
Signed-off-by: Sean Christopherson <[e-mail]>
---
 arch/x86/kvm/cpuid.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)

Patch

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index bfb81e417bef..c7fb6b764075 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -579,18 +579,37 @@  int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
 	return 0;
 }
 
+static __always_inline u32 raw_cpuid_get(struct cpuid_reg cpuid)
+{
+	struct kvm_cpuid_entry2 entry;
+	u32 base;
+
+	/*
+	 * KVM only supports features defined by Intel (0x0), AMD (0x80000000),
+	 * and Centaur (0xc0000000).  WARN if a feature for new vendor base is
+	 * defined, as this and other code would need to be updated.
+	 */
+	base = cpuid.function & 0xffff0000;
+	if (WARN_ON_ONCE(base && base != 0x80000000 && base != 0xc0000000))
+		return 0;
+
+	if (cpuid_eax(base) < cpuid.function)
+		return 0;
+
+	cpuid_count(cpuid.function, cpuid.index,
+		    &entry.eax, &entry.ebx, &entry.ecx, &entry.edx);
+
+	return *__cpuid_entry_get_reg(&entry, cpuid.reg);
+}
+
 /* Mask kvm_cpu_caps for @leaf with the raw CPUID capabilities of this CPU. */
 static __always_inline void __kvm_cpu_cap_mask(unsigned int leaf)
 {
 	const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);
-	struct kvm_cpuid_entry2 entry;
 
 	reverse_cpuid_check(leaf);
 
-	cpuid_count(cpuid.function, cpuid.index,
-		    &entry.eax, &entry.ebx, &entry.ecx, &entry.edx);
-
-	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, cpuid.reg);
+	kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid);
 }
 
 static __always_inline