linux-kernel - Re: [PATCH] KVM: x86: Take a u64 when checking for a valid dr7 value

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Date:   Sat, 25 Jan 2020 23:49:39 -0800
From:   Krish Sadhukhan <krish.sadhukhan@...cle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] KVM: x86: Take a u64 when checking for a valid dr7 value


On 1/24/20 3:07 PM, Sean Christopherson wrote:
> Take a u64 instead of an unsigned long in kvm_dr7_valid() to fix a build
> warning on i386 due to right-shifting a 32-bit value by 32 when checking
> for bits being set in dr7[63:32].
>
> Alternatively, the warning could be resolved by rewriting the check to
> use an i386-friendly method, but taking a u64 fixes another oddity on
> 32-bit KVM.  Beause KVM implements natural width VMCS fields as u64s to
> avoid layout issues between 32-bit and 64-bit, a devious guest can stuff
> vmcs12->guest_dr7 with a 64-bit value even when both the guest and host
> are 32-bit kernels.  KVM eventually drops vmcs12->guest_dr7[63:32] when
> propagating vmcs12->guest_dr7 to vmcs02, but ideally KVM would not rely
> on that behavior for correctness.
>
> Cc: Jim Mattson <jmattson@...gle.com>
> Cc: Krish Sadhukhan <krish.sadhukhan@...cle.com>
> Fixes: ecb697d10f70 ("KVM: nVMX: Check GUEST_DR7 on vmentry of nested guests")
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>   arch/x86/kvm/x86.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 2d2ff855773b..3624665acee4 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -357,7 +357,7 @@ static inline bool kvm_pat_valid(u64 data)
>   	return (data | ((data & 0x0202020202020202ull) << 1)) == data;
>   }
>   
> -static inline bool kvm_dr7_valid(unsigned long data)
> +static inline bool kvm_dr7_valid(u64 data)
>   {
>   	/* Bits [63:32] are reserved */
>   	return !(data >> 32);
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@...cle.com>