Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0

[Like Xu <like.xu.linux@gmail.com>]

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
>