linux-kernel - Re: [PATCH] KVM: x86: fix CPUID entries returned by KVM_GET

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Message-ID: <87a6st31c6.fsf@vitty.brq.redhat.com>
Date:   Thu, 28 Jan 2021 09:36:09 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org,
        Michael Roth <michael.roth@....com.com>
Subject: Re: [PATCH] KVM: x86: fix CPUID entries returned by KVM_GET_CPUID2
 ioctl

Michael Roth <michael.roth@....com> writes:

> Recent commit 255cbecfe0 modified struct kvm_vcpu_arch to make
> 'cpuid_entries' a pointer to an array of kvm_cpuid_entry2 entries
> rather than embedding the array in the struct. KVM_SET_CPUID and
> KVM_SET_CPUID2 were updated accordingly, but KVM_GET_CPUID2 was missed.
>
> As a result, KVM_GET_CPUID2 currently returns random fields from struct
> kvm_vcpu_arch to userspace rather than the expected CPUID values. Fix
> this by treating 'cpuid_entries' as a pointer when copying its
> contents to userspace buffer.
>
> Fixes: 255cbecfe0c9 ("KVM: x86: allocate vcpu->arch.cpuid_entries dynamically")
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Michael Roth <michael.roth@....com.com>
> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 13036cf0b912..38172ca627d3 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -321,7 +321,7 @@ int kvm_vcpu_ioctl_get_cpuid2(struct kvm_vcpu *vcpu,
>  	if (cpuid->nent < vcpu->arch.cpuid_nent)
>  		goto out;
>  	r = -EFAULT;
> -	if (copy_to_user(entries, &vcpu->arch.cpuid_entries,
> +	if (copy_to_user(entries, vcpu->arch.cpuid_entries,
>  			 vcpu->arch.cpuid_nent * sizeof(struct kvm_cpuid_entry2)))
>  		goto out;
>  	return 0;

This is embarrassing but I have a (possible) excuse: copy_to_user's
argument is 'void *' so no warning was produced. Surprisingly, no test
caught the breakage. Thanks for debugging and fixing!

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

-- 
Vitaly