linux-kernel - Re: [PATCH 05/61] KVM: x86: Check userapce CPUID array size after validating sub-leaf

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Message-ID: <87k14zg2m7.fsf@vitty.brq.redhat.com>
Date:   Thu, 06 Feb 2020 16:24:48 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 05/61] KVM: x86: Check userapce CPUID array size after validating sub-leaf

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Verify that the next sub-leaf of CPUID 0x4 (or 0x8000001d) is valid
> before rejecting the entire KVM_GET_SUPPORTED_CPUID due to insufficent
> space in the 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 the affected leafs.  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@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 11d5f311ef10..e5cf1e0cf84a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -552,12 +552,12 @@ static inline int __do_cpuid_func(struct kvm_cpuid_entry2 *entry, u32 function,
>  
>  		/* read more entries until cache_type is zero */
>  		for (i = 1; ; ++i) {
> -			if (*nent >= maxnent)
> -				goto out;
> -
>  			cache_type = entry[i - 1].eax & 0x1f;
>  			if (!cache_type)
>  				break;
> +
> +			if (*nent >= maxnent)
> +				goto out;
>  			do_host_cpuid(&entry[i], function, i);
>  			++*nent;
>  		}

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly