On Wed, May 31, 2023, Jon Kohler wrote: > > > On May 31, 2023, at 12:30 PM, Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > Caveat #1: cpu_feature_enabled() has a flaw that's relevant to this code: in the > > unlikely scenario that the compiler doesn't resolve "cid" to a compile-time > > constant value, cpu_feature_enabled() won't query DISABLED_MASK_BIT_SET(). I don't > > see any other use of cpu_feature_enabled() without a hardcoded X86_FEATURE_*, and > > the below compiles with my config, so I think/hope we can just require a compile-time > > constant when using cpu_feature_enabled(). > > > > Yea I think that should work. I’ll club that into v2 of this patch. I recommend doing it as a separate patch, hardening cpu_feature_enabled() shouldn't have a dependency on tweaking the xfeatures mask. I tested this with an allyesconfig if you want to throw it in as a prep patch. --- From: Sean Christopherson <seanjc@xxxxxxxxxx> Date: Wed, 31 May 2023 09:41:12 -0700 Subject: [PATCH] x86/cpufeature: Require compile-time constant in cpu_feature_enabled() Assert that the to-be-checked bit passed to cpu_feature_enabled() is a compile-time constant instead of applying the DISABLED_MASK_BIT_SET() logic if and only if the bit is a constant. Conditioning the check on the bit being constant instead of requiring the bit to be constant could result in compiler specific kernel behavior, e.g. running on hardware that supports a disabled feature would return %false if the compiler resolved the bit to a constant, but %true if not. All current usage of cpu_feature_enabled() specifies a hardcoded X86_FEATURE_* flag, so this *should* be a glorified nop. Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> --- arch/x86/include/asm/cpufeature.h | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h index ce0c8f7d3218..886200fbf8d9 100644 --- a/arch/x86/include/asm/cpufeature.h +++ b/arch/x86/include/asm/cpufeature.h @@ -141,8 +141,11 @@ extern const char * const x86_bug_flags[NBUGINTS*32]; * supporting a possible guest feature where host support for it * is not relevant. */ -#define cpu_feature_enabled(bit) \ - (__builtin_constant_p(bit) && DISABLED_MASK_BIT_SET(bit) ? 0 : static_cpu_has(bit)) +#define cpu_feature_enabled(bit) \ +({ \ + BUILD_BUG_ON(!__builtin_constant_p(bit)); \ + DISABLED_MASK_BIT_SET(bit) ? 0 : static_cpu_has(bit); \ +}) #define boot_cpu_has(bit) cpu_has(&boot_cpu_data, bit) base-commit: 39428f6ea9eace95011681628717062ff7f5eb5f -- > > I'm totally ok gating xfeature bits on cpu_feature_enabled(), but there should be > > a prep patch for KVM to clear features bits in kvm_cpu_caps if the corresponding > > XCR0/XSS bit is not set in the host. If KVM ever wants to expose an xstate feature > > (other than MPX) that's disabled in the host, then we can revisit > > fpu__init_system_xstate(). But we need to ensure the "failure" mode is that > > KVM doesn't advertise the feature, as opposed to advertising a feature without > > without context switching its data. > > > Looking into this, trying to understand the comment about a feature being used > without context switching its data. > > In __kvm_x86_vendor_init() we’re already populating host_xcr0 using the > XCR_XFEATURE_ENABLED_MASK, which should be populated on boot > by fpu__init_cpu_xstate(), which happens almost immediately after the code that I > modified in this commit. > > That then flows into guest_supported_xcr0 (as well as user_xfeatures). > guest_supported_xcr0 is then plumbed into __kvm_set_xcr, which specifically says > that we’re using that to prevent the guest from setting bits that we won’t save in the > first place. > > /* > * Do not allow the guest to set bits that we do not support > * saving. However, xcr0 bit 0 is always set, even if the > * emulated CPU does not support XSAVE (see kvm_vcpu_reset()). > */ > valid_bits = vcpu->arch.guest_supported_xcr0 | XFEATURE_MASK_FP; > > Wouldn’t this mean that the *guest* xstate initialization would not see a given > feature too and take care of the problem naturally? > > Or are you saying you’d want an even more detailed clearing? The CPUID bits that enumerate support for a feature are independent from the CPUID bits that enumerate what XCR0 bits are supported, i.e. what features can be saved and restored via XSAVE/XRSTOR. KVM does mostly account for host XCR0, but in a very ad hoc way. E.g. MPX is handled by manually checking host XCR0. if (kvm_mpx_supported()) kvm_cpu_cap_check_and_set(X86_FEATURE_MPX); PKU manually checks too, but indirectly by looking at whether or not the kernel has enabled CR4.OSPKE. if (!tdp_enabled || !boot_cpu_has(X86_FEATURE_OSPKE)) kvm_cpu_cap_clear(X86_FEATURE_PKU); But unless I'm missing something, the various AVX and AMX bits rely solely on boot_cpu_data, i.e. would break if someone added CONFIG_X86_AVX or CONFIG_X86_AMX.