linux-kernel - Re: [PATCH 10/27] KVM: MMU: Add infrastructure for two-level page walker

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Message-ID: <4C852D6F.5000101@redhat.com>
Date:	Mon, 06 Sep 2010 21:05:35 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Joerg Roedel <joerg.roedel@....com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Alexander Graf <agraf@...e.de>, joro@...tes.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/27] KVM: MMU: Add infrastructure for two-level page
 walker

  On 09/06/2010 06:55 PM, Joerg Roedel wrote:
> This patch introduces a mmu-callback to translate gpa
> addresses in the walk_addr code. This is later used to
> translate l2_gpa addresses into l1_gpa addresses.

> @@ -534,6 +534,11 @@ static inline gpa_t gfn_to_gpa(gfn_t gfn)
>   	return (gpa_t)gfn<<  PAGE_SHIFT;
>   }
>
> +static inline gfn_t gpa_to_gfn(gpa_t gpa)
> +{
> +	return (gfn_t)gpa>>  PAGE_SHIFT;
> +}
> +

That's a bug - gfn_t may be smaller than gpa_t, so you're truncating 
just before the shift.  Note the casts in the surrounding functions are 
widening, not narrowing.

However, gfn_t is u64 so the bug is only theoretical.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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