On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> Add compile-time assertions to verify that usage of F() and friends in
> kvm_set_cpu_caps() is scoped to the correct CPUID word, e.g. to detect
> bugs where KVM passes a feature bit from word X into word y.
>
> Add a one-off assertion in the aliased feature macro to ensure that only
> word 0x8000_0001.EDX aliased the features defined for 0x1.EDX.
>
> To do so, convert kvm_cpu_cap_init() to a macro and have it define a
> local variable to track which CPUID word is being initialized that is
> then used to validate usage of F() (all of the inputs are compile-time
> constants and thus can be fed into BUILD_BUG_ON()).
>
> Redefine KVM_VALIDATE_CPU_CAP_USAGE after kvm_set_cpu_caps() to be a nop
> so that F() can be used in other flows that aren't as easily hardened,
> e.g. __do_cpuid_func_emulated() and __do_cpuid_func().
>
> Invoke KVM_VALIDATE_CPU_CAP_USAGE() in SF() and X86_64_F() to ensure the
> validation occurs, e.g. if the usage of F() is completely compiled out
> (which shouldn't happen for boot_cpu_has(), but could happen in the future,
> e.g. if KVM were to use cpu_feature_enabled()).
>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
> arch/x86/kvm/cpuid.c | 55 +++++++++++++++++++++++++++++++-------------
> 1 file changed, 39 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index a16d6e070c11..1064e4d68718 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -61,18 +61,24 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
> return ret;
> }
>
> -#define F feature_bit
> +#define F(name) \
> +({ \
> + KVM_VALIDATE_CPU_CAP_USAGE(name); \
> + feature_bit(name); \
> +})
>
> /* Scattered Flag - For features that are scattered by cpufeatures.h. */
> #define SF(name) \
> ({ \
> BUILD_BUG_ON(X86_FEATURE_##name >= MAX_CPU_FEATURES); \
> + KVM_VALIDATE_CPU_CAP_USAGE(name); \
> (boot_cpu_has(X86_FEATURE_##name) ? F(name) : 0); \
> })
>
> /* Features that KVM supports only on 64-bit kernels. */
> #define X86_64_F(name) \
> ({ \
> + KVM_VALIDATE_CPU_CAP_USAGE(name); \
> (IS_ENABLED(CONFIG_X86_64) ? F(name) : 0); \
> })
>
> @@ -95,6 +101,7 @@ u32 xstate_required_size(u64 xstate_bv, bool compacted)
> #define AF(name) \
> ({ \
> BUILD_BUG_ON(__feature_leaf(X86_FEATURE_##name) != CPUID_1_EDX); \
> + BUILD_BUG_ON(kvm_cpu_cap_init_in_progress != CPUID_8000_0001_EDX); \
> feature_bit(name); \
> })
>
> @@ -622,22 +629,34 @@ static __always_inline u32 raw_cpuid_get(struct cpuid_reg cpuid)
> return *__cpuid_entry_get_reg(&entry, cpuid.reg);
> }
>
> -static __always_inline void kvm_cpu_cap_init(u32 leaf, u32 mask)
> -{
> - const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);
> +/*
> + * Assert that the feature bit being declared, e.g. via F(), is in the CPUID
> + * word that's being initialized. Exempt 0x8000_0001.EDX usage of 0x1.EDX
> + * features, as AMD duplicated many 0x1.EDX features into 0x8000_0001.EDX.
> + */
> +#define KVM_VALIDATE_CPU_CAP_USAGE(name) \
> +do { \
> + u32 __leaf = __feature_leaf(X86_FEATURE_##name); \
> + \
> + BUILD_BUG_ON(__leaf != kvm_cpu_cap_init_in_progress); \
> +} while (0)
>
> - /*
> - * For kernel-defined leafs, mask the boot CPU's pre-populated value.
> - * For KVM-defined leafs, explicitly set the leaf, as KVM is the one
> - * and only authority.
> - */
> - if (leaf < NCAPINTS)
> - kvm_cpu_caps[leaf] &= mask;
> - else
> - kvm_cpu_caps[leaf] = mask;
> -
> - kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid);
> -}
> +/*
> + * For kernel-defined leafs, mask the boot CPU's pre-populated value. For KVM-
> + * defined leafs, explicitly set the leaf, as KVM is the one and only authority.
> + */
> +#define kvm_cpu_cap_init(leaf, mask) \
> +do { \
> + const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32); \
> + const u32 __maybe_unused kvm_cpu_cap_init_in_progress = leaf; \
Why not to #define the kvm_cpu_cap_init_in_progress as well instead of a variable?
> + \
> + if (leaf < NCAPINTS) \
> + kvm_cpu_caps[leaf] &= (mask); \
> + else \
> + kvm_cpu_caps[leaf] = (mask); \
> + \
> + kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid); \
> +} while (0)
>
> /*
> * Undefine the MSR bit macro to avoid token concatenation issues when
> @@ -870,6 +889,10 @@ void kvm_set_cpu_caps(void)
> }
> EXPORT_SYMBOL_GPL(kvm_set_cpu_caps);
>
> +#undef kvm_cpu_cap_init
> +#undef KVM_VALIDATE_CPU_CAP_USAGE
> +#define KVM_VALIDATE_CPU_CAP_USAGE(name)
> +
> struct kvm_cpuid_array {
> struct kvm_cpuid_entry2 *entries;
> int maxnent;
Best regards,
Maxim Levitsky
