linux-kernel - Re: [PATCH] x86: NX protection for kernel data

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Date:	Sun, 19 Jul 2009 20:09:14 -0400
From:	Siarhei Liakh <sliakh.lkml@...il.com>
To:	Sam Ravnborg <sam@...nborg.org>
Cc:	linux-kernel@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, 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>, linux-cris-kernel@...s.com,
	Roland Dreier <rdreier@...co.com>
Subject: Re: [PATCH] x86: NX protection for kernel data

On Sun, Jul 19, 2009 at 4:13 PM, Sam Ravnborg<sam@...nborg.org> wrote:
> On Sun, Jul 19, 2009 at 03:43:06PM -0400, Siarhei Liakh wrote:
>> This patch expands functionality of CONFIG_DEBUG_RODATA to set main
>> (static) kernel data area as NX.
>> The following steps are taken to achieve this:
>> 1. Linker scripts are adjusted so .text always starts and end on a page boundary
>> 2. Linker scripts are adjusted so .rodata and .data always starts and
>> end on a page boundary
>> 3. void mark_nxdata_nx(void) added to arch/x86/mm/init_64.c and
>> arch/x86/mm/init_32.c with actual functionality: NX is set for all
>> pages from _etext through _edata
>> 4. mark_nxdata_nx() called from init_post(void) in init/main.c
>>
>> The patch have been developed for Linux 2.6.30 x86 by Siarhei Liakh
>> <sliakh.lkml@...il.com> and Xuxian Jiang <jiang@...ncsu.edu>.
>
>
> The patch no longer applies.
> The file vmlinux_32.lds.S and vmlinux_64.lds.S has been unified
> into one file.

That is actually a great news. I will get the latest source and
re-write the patch.

>> --- a/arch/x86/kernel/vmlinux_32.lds.S
>> +++ b/arch/x86/kernel/vmlinux_32.lds.S
>> @@ -47,6 +47,7 @@ SECTIONS
>>       IRQENTRY_TEXT
>>       *(.fixup)
>>       *(.gnu.warning)
>> +     . = ALIGN(PAGE_SIZE);   /* .text should occupy whole number of pages */
>>       _etext = .;                     /* End of text section */
>
> So _etext cover until page boundary - makes sense.
>
>>    } :text = 0x9090
>>
>> @@ -93,6 +94,7 @@ SECTIONS
>>       *(.data.read_mostly)
>>       _edata = .;             /* End of data section */
>>    }
>> +  . = ALIGN(PAGE_SIZE);              /* needed so we can set NX for .data */
>
> But here _edata does not cover until page boundary.
> And alignmnet is located outside the output section
> definition.
> It would be better/more consistent to follow the style you use for .text here.

You are correct. _edata should be the last thing in .data, and
alignment should be done before it. However, this brings up a
question: was there any specific reason to leave .data.init_task
beyond the _edata? Should we move _edata into the the last of the
.data.* sections to have poper view of kernel layout?
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