Re: [PATCH v2 2/3] KVM: x86: Harden against unexpected adjustments to kvm_cpu_caps

[Binbin Wu <binbin.wu@linux.intel.com>]

On 1/28/2026 9:43 AM, Sean Christopherson wrote:
> 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: Binbin Wu <binbin.wu@linux.intel.com>

> Signed-off-by: Sean Christopherson <seanjc@google.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,