Re: [PATCH 07/61] KVM: x86: Check for CPUID 0xD.N support before validating array size

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> Now that sub-leaf 1 is handled separately, verify the next sub-leaf is
> needed before rejecting KVM_GET_SUPPORTED_CPUID due to an insufficiently
> sized userspace array.
>
> Note, although this is technically a bug, it's not visible to userspace
> as KVM_GET_SUPPORTED_CPUID is guaranteed to fail on KVM_CPUID_SIGNATURE,
> which is hardcoded to be added after leaf 0xD.  The real motivation for
> the change is to tightly couple the nent/maxnent and do_host_cpuid()
> sequences in preparation for future cleanup.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index fc8540596386..fd9b29aa7abc 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -670,13 +670,14 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  		entry[1].edx = 0;
>  
>  		for (idx = 2, i = 2; idx < 64; ++idx) {
> -			u64 mask = ((u64)1 << idx);
> +			if (!(supported & BIT_ULL(idx)))
> +				continue;
>  
>  			if (*nent >= maxnent)
>  				goto out;
>  
>  			do_host_cpuid(&entry[i], function, idx);
> -			if (entry[i].eax == 0 || !(supported & mask))
> +			if (entry[i].eax == 0)
>  				continue;
>  			if (WARN_ON_ONCE(entry[i].ecx & 1))
>  				continue;

The remaining WARN_ON_ONCE() is technically the same 'bug not visible to
userspace' :-)

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

-- 
Vitaly