Re: [tip:x86/security] x86: Add NX protection for kernel data

[Konrad Rzeszutek Wilk]

On Tue, Jan 11, 2011 at 03:31:35PM -0800, Kees Cook wrote:
> On Thu, Nov 18, 2010 at 02:08:22PM +0000, tip-bot for Matthieu Castet wrote:
> > Commit-ID:  5bd5a452662bc37c54fb6828db1a3faf87e6511c
> > Gitweb:     http://git.kernel.org/tip/5bd5a452662bc37c54fb6828db1a3faf87e6511c
> > Author:     Matthieu Castet <castet.matthieu@xxxxxxx>
> > AuthorDate: Tue, 16 Nov 2010 22:31:26 +0100
> > Committer:  Ingo Molnar <mingo@xxxxxxx>
> > CommitDate: Thu, 18 Nov 2010 12:52:04 +0100
> > 
> > x86: Add NX protection for kernel data
> 
> [I'd sent this in reply to the wrong patch before, resending now...]
> 
> Hi,
> 
> I was just shown this[1] on Xen from an Ubuntu bug report[2].
> 
> [    1.230382] NX-protecting the kernel data: 3884k
> [    1.231002] BUG: unable to handle kernel paging request at c1782ae0
> ...
> [    1.231145] Call Trace:
> [    1.231152]  [<c0138481>] ? __change_page_attr+0x2c1/0x370
> [    1.231161]  [<c02163a1>] ? __purge_vmap_area_lazy+0xc1/0x180
> [    1.231169]  [<c013857c>] ? __change_page_attr_set_clr+0x4c/0xb0
> [    1.231176]  [<c0138838>] ? change_page_attr_set_clr+0x128/0x300
> [    1.231183]  [<c010798e>] ? __raw_callee_save_xen_restore_fl+0x6/0x8
> [    1.231192]  [<c0159ca1>] ? vprintk+0x171/0x3f0
> [    1.231198]  [<c0138bdf>] ? set_memory_nx+0x5f/0x70

If you run it with Xen debugging enabled:

[    7.753329] NX-protecting the kernel data: 2400k
(XEN) mm.c:2389:d0 Bad type (saw 3c000003 != exp 70000000) for mfn 1355a5 (pfn 15a5)
(XEN) mm.c:889:d0 Error getting mfn 1355a5 (pfn 15a5) from L1 entry 80000001355a5063 for l1e_owner=0, pg_owner=0
(XEN) mm.c:4958:d0 ptwr_emulate: could not get_page_from_l1e()
[    7.759087] BUG: unable to handle kernel paging request at c82a4d28
[    7.759087] IP: [<c100608c>] xen_set_pte_atomic+0x21/0x2f
[    7.759087] *pdpt = 0000000001663001 *pde = 00000000082db067 *pte = 80000000082a4061 
.. and same stack trace.

> 
> 
> Does Xen have different size page table allocations or something weird?

The same page size. Not sure actually why it is being triggered. Let me copy
Keir on this. Keir, the region that is being marked as _NX is .bss one and
they are marked as RWE in the ELF phhdrs.

Reverting above git 5bd5a452662bc37c54fb6828db1a3faf87e6511c does make Linux
work under Xen.

> 
> -Kees
> 
> [1] http://launchpadlibrarian.net/61853131/natty-failed-boot-ec2.txt
> [2] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/699828
> 
> -- 
> Kees Cook
> Ubuntu Security Team
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