linux-kernel - Re: [PATCH] xen/setup: add paranoid index check and warning

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Message-ID: <543CEB56.3080009@citrix.com>
Date:	Tue, 14 Oct 2014 10:22:30 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Martin Kelly <martin@...tingkelly.com>,
	<xen-devel@...ts.xenproject.org>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>
CC:	<konrad.wilk@...cle.com>, <boris.ostrovsky@...cle.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>,
	Martin Kelly <martkell@...zon.com>
Subject: Re: [PATCH] xen/setup: add paranoid index check and warning

On 14/10/14 02:19, Martin Kelly wrote:
> In a call to set_phys_range_identity, i-1 is used without checking that
> i is non-zero. Although unlikely, a bug in the code before it could
> cause the value to be 0, leading to erroneous behavior. This patch adds
> a check against 0 value and a corresponding warning.

This can only happen if the toolstack supplies a memory map with zero
entries.  I could see justification for adding a panic at the top of
this function in this case, but I can't see the usefulness of this warning.

> --- a/arch/x86/xen/setup.c
> +++ b/arch/x86/xen/setup.c
> @@ -671,7 +671,10 @@ char * __init xen_memory_setup(void)
>  	 * PFNs above MAX_P2M_PFN are considered identity mapped as
>  	 * well.
>  	 */
> -	set_phys_range_identity(map[i-1].addr / PAGE_SIZE, ~0ul);
> +	if (i > 0)
> +		set_phys_range_identity(map[i-1].addr / PAGE_SIZE, ~0ul);
> +	else
> +		WARN(1, "Something went wrong clamping memory to a factor of EXTRA_MEM_RATIO!");
>  
>  	/*
>  	 * In domU, the ISA region is normal, usable memory, but we
> 

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