linux-kernel - [RFC] reworked NX protection for kernel data

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Message-ID: <4C6474AA.4040802@free.fr>
Date:	Fri, 13 Aug 2010 00:24:42 +0200
From:	matthieu castet <castet.matthieu@...e.fr>
To:	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Siarhei Liakh <sliakh.lkml@...il.com>
CC:	Xuxian Jiang <jiang@...ncsu.edu>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	James Morris <jmorris@...ei.org>
Subject: [RFC] reworked NX protection for kernel data

Hi,

because nobody seems to work on "NX protection for kernel data" patches [1], I decided to rework them a bit :
- remove unneeded alignment in vmlinux.lds
- call mark_nxdata_nx in mark_rodata_ro instead of free_initmem
- fix a bug in my machine where memory was x and rw after kernel data
- static_protections for bios rom is only enabled when CONFIG_PCI_BIOS (no x64)
- bios rom is set to x only when probing pcibios (this won't be done is mmconfig is supported).

A second patch is more radical : it make the ROM bios memory (0xe0000-0x100000) ro and x, but only for system
supporting NX (to avoid breaking very old machine). I don't know if it is safe.

I am waiting for your comment before reposting all the patches (module and improper large page preservation)
I only test it on a 32bit NX cpu.


Matthieu

[1]
Note: this patch depends on "Correct improper large page preservation" patch

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 script is adjusted so .text always starts and ends on a page bound
2. Linker script is adjusted so .rodata and .data always start and
end on a page boundary
3. void mark_nxdata_nx(void) added to arch/x86/mm/init.c with actual
functionality: NX is set for all pages from _etext through _end.
4. mark_nxdata_nx() called from free_initmem() (after init has been released
5. free_init_pages() sets released memory NX in arch/x86/mm/init.c

The results of patch application may be observed in the diff of kernel page
table dumps:
--- data_nx_pt_before.txt       2009-10-13 07:48:59.000000000 -0400
+++ data_nx_pt_after.txt        2009-10-13 07:26:46.000000000 -0400
@@ -2,8 +2,9 @@
 0x00000000-0xc0000000           3G                           pmd
 ---[ Kernel Mapping ]---
 0xc0000000-0xc0100000           1M     RW             GLB x  pte
-0xc0100000-0xc048d000        3636K     ro             GLB x  pte
-0xc048d000-0xc0600000        1484K     RW             GLB x  pte
+0xc0100000-0xc0381000        2564K     ro             GLB x  pte
+0xc0381000-0xc048d000        1072K     ro             GLB NX pte
+0xc048d000-0xc0600000        1484K     RW             GLB NX pte
 0xc0600000-0xf7800000         882M     RW         PSE GLB NX pmd
 0xf7800000-0xf79fe000        2040K     RW             GLB NX pte
 0xf79fe000-0xf7a00000           8K                           pte

The patch have been developed for Linux 2.6.34-rc2 x86 by Siarhei Liakh
<sliakh.lkml@...il.com> and Xuxian Jiang <jiang@...ncsu.edu>.

V1:  initial patch for 2.6.30
V2:  patch for 2.6.31-rc7
V3:  moved all code into arch/x86, adjusted credits
V4:  fixed ifdef, removed credits from CREDITS
V5:  fixed an address calculation bug in mark_nxdata_nx()
V6:  added acked-by and PT dump diff to commit log
V7:  minor adjustments for -tip

Signed-off-by: Siarhei Liakh <sliakh.lkml@...il.com>
Signed-off-by: Xuxian Jiang <jiang@...ncsu.edu>
Acked-by: Arjan van de Ven <arjan@...ux.intel.com>
Reviewed-by: James Morris <jmorris@...ei.org>

View attachment "kernelnx.diff" of type "text/x-diff" (5368 bytes)

View attachment "kernelnx2.diff" of type "text/x-diff" (2460 bytes)