linux-kernel - Re: [PATCH 1/4] [tip:x86/mm] Correcting improper large page preservation

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Date:	Fri, 02 Apr 2010 23:43:20 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Siarhei Liakh <sliakh.lkml@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-security-module@...r.kernel.org" 
	<linux-security-module@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	James Morris <jmorris@...ei.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@....de>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Dave Jones <davej@...hat.com>
Subject: Re: [PATCH 1/4] [tip:x86/mm] Correcting improper large page
 preservation

On Wed, 2010-03-31 at 18:59 -0700, Siarhei Liakh wrote:
> +	/*
> +	 * .data and .bss should always be writable.
> +	 */
> +	if ((within(pfn, __pa((unsigned long)_sdata) >> PAGE_SHIFT,
> +		   __pa((unsigned long)_edata) >> PAGE_SHIFT)) ||
> +	    (within(pfn, __pa((unsigned long)__bss_start) >> PAGE_SHIFT,
> +		   __pa((unsigned long)__bss_stop) >> PAGE_SHIFT))) {
> +		pgprot_val(required) |= _PAGE_RW;
> +	}

I have reviewed this patch and the only comment I have is:

On 64bit kernels, kernel text/data mapping and kernel identity mappings
are different virtual addresses mapping to same pfn ranges. For the
data/bss pages, does it help (in identifying certain data corruptions
more easily) in making the kernel identity mapping to be set to
read-only and enforce the need of RW only for the kernel data mappings.

Or is there some obscure code that uses something like
__va(__pa(data_symbol)) and writes to it?

If not, we can remove the __pa() constructs above and use the addr for
comparisons.

Otherwise this patch looks good to me.

Reviewed-by: Suresh Siddha <suresh.b.siddha@...el.com>

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