From a7c48c3f56db8d18d15ee6d8c2c5e2da447c77cc Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Tue, 17 Dec 2019 13:32:41 -0800 Subject: [PATCH] KVM: x86: Expand build-time assertion on reverse CPUID usage Add build-time checks to ensure KVM isn't trying to do a reverse CPUID lookup on Linux-defined feature bits, along with comments to explain the gory details of X86_FEATUREs and bit(). No functional change intended. Signed-off-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- arch/x86/kvm/cpuid.c | 3 ++- arch/x86/kvm/cpuid.h | 33 ++++++++++++++++++++++++++++----- 2 files changed, 30 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index cf55629ff0ff..ca20951fac2b 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -281,8 +281,9 @@ out: return r; } -static void cpuid_mask(u32 *word, int wordnum) +static __always_inline void cpuid_mask(u32 *word, int wordnum) { + reverse_cpuid_check(wordnum); *word &= boot_cpu_data.x86_capability[wordnum]; } diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h index 5fc424d2e1fa..fb3c44b019ec 100644 --- a/arch/x86/kvm/cpuid.h +++ b/arch/x86/kvm/cpuid.h @@ -56,18 +56,41 @@ static const struct cpuid_reg reverse_cpuid[] = { [CPUID_7_1_EAX] = { 7, 1, CPUID_EAX}, }; -static inline u32 bit(int bitno) +/* + * Reverse CPUID and its derivatives can only be used for hardware-defined + * feature words, i.e. words whose bits directly correspond to a CPUID leaf. + * Retrieving a feature bit or masking guest CPUID from a Linux-defined word + * is nonsensical as the bit number/mask is an arbitrary software-defined value + * and can't be used by KVM to query/control guest capabilities. And obviously + * the leaf being queried must have an entry in the lookup table. + */ +static __always_inline void reverse_cpuid_check(unsigned x86_leaf) { - return BIT(bitno & 31); + BUILD_BUG_ON(x86_leaf == CPUID_LNX_1); + BUILD_BUG_ON(x86_leaf == CPUID_LNX_2); + BUILD_BUG_ON(x86_leaf == CPUID_LNX_3); + BUILD_BUG_ON(x86_leaf == CPUID_LNX_4); + BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid)); + BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0); +} + +/* + * Retrieve the bit mask from an X86_FEATURE_* definition. Features contain + * the hardware defined bit number (stored in bits 4:0) and a software defined + * "word" (stored in bits 31:5). The word is used to index into arrays of + * bit masks that hold the per-cpu feature capabilities, e.g. this_cpu_has(). + */ +static __always_inline u32 bit(int x86_feature) +{ + reverse_cpuid_check(x86_feature / 32); + return 1 << (x86_feature & 31); } static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature) { unsigned x86_leaf = x86_feature / 32; - BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid)); - BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0); - + reverse_cpuid_check(x86_leaf); return reverse_cpuid[x86_leaf]; } -- 2.39.5