Messages in this thread | | | Date | Mon, 24 Jan 2022 15:17:39 +0800 | Subject | Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 | From | Like Xu <> |
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On 24/1/2022 3:06 pm, Tian, Kevin wrote: >> From: Like Xu <like.xu.linux@gmail.com> >> Sent: Sunday, January 23, 2022 1:50 PM >> >> From: Like Xu <likexu@tencent.com> >> >> A malicious user space can bypass xstate_get_guest_group_perm() in the >> KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures, >> since the validity check of xcr0 depends only on guest_supported_xcr0. > > Unpermitted xfeatures cannot pass kvm_check_cpuid()...
Indeed, 5ab2f45bba4894a0db4af8567da3efd6228dd010.
This part of logic is pretty fragile and fragmented due to semantic inconsistencies between supported_xcr0 and guest_supported_xcr0 in other three places:
- __do_cpuid_func - kvm_mpx_supported - kvm_vcpu_ioctl_x86_set_xsave
Have you identified all their areas of use ?
> >> >> Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at >> KVM_GET_SUPPORTED_CPUID") >> Signed-off-by: Like Xu <likexu@tencent.com> >> --- >> arch/x86/kvm/cpuid.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >> index 3902c28fb6cb..1bd4d560cbdd 100644 >> --- a/arch/x86/kvm/cpuid.c >> +++ b/arch/x86/kvm/cpuid.c >> @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct >> kvm_vcpu *vcpu) >> vcpu->arch.guest_supported_xcr0 = 0; >> else >> vcpu->arch.guest_supported_xcr0 = >> - (best->eax | ((u64)best->edx << 32)) & >> supported_xcr0; >> + (best->eax | ((u64)best->edx << 32)) & >> + (supported_xcr0 & xstate_get_guest_group_perm()); >> >> /* >> * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1) >> enumerate >> -- >> 2.33.1 >
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