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KVM: x86: Harden CPU capabilities processing against out-of-scope features

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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()).

Link: https://lore.kernel.org/r/20241128013424.4096668-29-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2024-11-27 17:33:55 -08:00
parent 8d862c270b
commit 3d142340d7
1 changed files with 35 additions and 16 deletions
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51
arch/x86/kvm/cpuid.c
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@@ -623,35 +623,53 @@ 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);
/*
* 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; \
\
if (leaf < NCAPINTS) \
kvm_cpu_caps[leaf] &= (mask); \
else \
kvm_cpu_caps[leaf] = (mask); \
\
kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid); \
} 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;
/*
* 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)
kvm_cpu_caps[leaf] &= raw_cpuid_get(cpuid);
}
#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); \
})
@@ -662,6 +680,7 @@ static __always_inline void kvm_cpu_cap_init(u32 leaf, u32 mask)
#define ALIASED_1_EDX_F(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); \
})