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.

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://patch.msgid.link/20260128014310.3255561-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 0172b7ef2ba405893857fc155600f4f5adb848a2..3508fb8606e9a058c0abf13c2e9f7ec7e930b12f 100644 (file)
--- 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));
 
@@ -1288,7 +1294,7 @@ 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 d3f5ae15a7ca314675ad0b4e7347ddd2c205a69f..039b8e6f40baf77d5d38aefcad6f958203bf780e 100644 (file)
--- 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 24d59ccfa40d9a79eb1d570207fa5debdff5a2fa..8cf1a47304dfd4e24a79670379334562d092f07d 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5202,7 +5202,7 @@ 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;
 
@@ -5284,6 +5284,8 @@ static __init void svm_set_cpu_caps(void)
         */
        kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
        kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
+
+       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 6b96f7aea20bd3f39389da8c9a1cf890d2e29b05..30fa20f8b72d0b55ba4092d11f849c65efe061fd 100644 (file)
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7994,7 +7994,7 @@ 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)
@@ -8051,6 +8051,8 @@ static __init void vmx_set_cpu_caps(void)
                kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
                kvm_cpu_cap_clear(X86_FEATURE_IBT);
        }
+
+       kvm_finalize_cpu_caps();
 }
 
 static bool vmx_is_io_intercepted(struct kvm_vcpu *vcpu,