linux-kernel - [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu

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Message-ID: <20260128014310.3255561-3-seanjc@google.com>
Date: Tue, 27 Jan 2026 17:43:09 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Chao Gao <chao.gao@...el.com>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Jim Mattson <jmattson@...gle.com>
Subject: [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu_caps

Add a flag to track when KVM is actively configuring its CPU caps, and
WARN if a cap is set or cleared if KVM isn't in its configuration stage.
Modifying CPU caps after {svm,vmx}_set_cpu_caps() can be fatal to KVM, as
vendor setup code expects the CPU caps to be frozen at that point, e.g.
will do additional configuration based on the caps.

Rename kvm_set_cpu_caps() to kvm_initialize_cpu_caps() to pair with the
new "finalize", and to make it more obvious that KVM's CPU caps aren't
fully configured within the function.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c   | 10 ++++++++--
 arch/x86/kvm/cpuid.h   | 12 +++++++++++-
 arch/x86/kvm/svm/svm.c |  4 +++-
 arch/x86/kvm/vmx/vmx.c |  4 +++-
 4 files changed, 25 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 575244af9c9f..7fe4e58a6ebf 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -36,6 +36,9 @@
 u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_cpu_caps);
 
+bool kvm_is_configuring_cpu_caps __read_mostly;
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_is_configuring_cpu_caps);
+
 struct cpuid_xstate_sizes {
 	u32 eax;
 	u32 ebx;
@@ -826,10 +829,13 @@ do {									\
 /* DS is defined by ptrace-abi.h on 32-bit builds. */
 #undef DS
 
-void kvm_set_cpu_caps(void)
+void kvm_initialize_cpu_caps(void)
 {
 	memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));
 
+	WARN_ON_ONCE(kvm_is_configuring_cpu_caps);
+	kvm_is_configuring_cpu_caps = true;
+
 	BUILD_BUG_ON(sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)) >
 		     sizeof(boot_cpu_data.x86_capability));
 
@@ -1289,7 +1295,7 @@ void kvm_set_cpu_caps(void)
 		kvm_cpu_cap_clear(X86_FEATURE_RDPID);
 	}
 }
-EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_set_cpu_caps);
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_initialize_cpu_caps);
 
 #undef F
 #undef SCATTERED_F
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index d3f5ae15a7ca..039b8e6f40ba 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -8,7 +8,15 @@
 #include <uapi/asm/kvm_para.h>
 
 extern u32 kvm_cpu_caps[NR_KVM_CPU_CAPS] __read_mostly;
-void kvm_set_cpu_caps(void);
+extern bool kvm_is_configuring_cpu_caps __read_mostly;
+
+void kvm_initialize_cpu_caps(void);
+
+static inline void kvm_finalize_cpu_caps(void)
+{
+	WARN_ON_ONCE(!kvm_is_configuring_cpu_caps);
+	kvm_is_configuring_cpu_caps = false;
+}
 
 void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu);
 struct kvm_cpuid_entry2 *kvm_find_cpuid_entry2(struct kvm_cpuid_entry2 *entries,
@@ -188,6 +196,7 @@ static __always_inline void kvm_cpu_cap_clear(unsigned int x86_feature)
 {
 	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
+	WARN_ON_ONCE(!kvm_is_configuring_cpu_caps);
 	kvm_cpu_caps[x86_leaf] &= ~__feature_bit(x86_feature);
 }
 
@@ -195,6 +204,7 @@ static __always_inline void kvm_cpu_cap_set(unsigned int x86_feature)
 {
 	unsigned int x86_leaf = __feature_leaf(x86_feature);
 
+	WARN_ON_ONCE(!kvm_is_configuring_cpu_caps);
 	kvm_cpu_caps[x86_leaf] |= __feature_bit(x86_feature);
 }
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index c00a696dacfc..5f0136dbdde6 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5305,7 +5305,7 @@ static __init void svm_adjust_mmio_mask(void)
 
 static __init void svm_set_cpu_caps(void)
 {
-	kvm_set_cpu_caps();
+	kvm_initialize_cpu_caps();
 
 	kvm_caps.supported_perf_cap = 0;
 
@@ -5389,6 +5389,8 @@ static __init void svm_set_cpu_caps(void)
 	kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
 
 	kvm_setup_xss_caps();
+
+	kvm_finalize_cpu_caps();
 }
 
 static __init int svm_hardware_setup(void)
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 9f85c3829890..93ec1e6181e4 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -8173,7 +8173,7 @@ static __init u64 vmx_get_perf_capabilities(void)
 
 static __init void vmx_set_cpu_caps(void)
 {
-	kvm_set_cpu_caps();
+	kvm_initialize_cpu_caps();
 
 	/* CPUID 0x1 */
 	if (nested)
@@ -8232,6 +8232,8 @@ static __init void vmx_set_cpu_caps(void)
 	}
 
 	kvm_setup_xss_caps();
+
+	kvm_finalize_cpu_caps();
 }
 
 static bool vmx_is_io_intercepted(struct kvm_vcpu *vcpu,
-- 
2.52.0.457.g6b5491de43-goog