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

[matthieu castet]

Le Wed, 19 Jan 2011 16:14:32 -0500,
Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> a écrit :

> On Fri, Jan 14, 2011 at 03:15:30PM -0500, Konrad Rzeszutek Wilk wrote:
> > 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
this happen if (x & (PGT_type_mask|PGT_pae_xen_l2)) != type)

but
#define PGT_type_mask       (7U<<29) /* Bits 29-31. */
#define _PGT_pae_xen_l2     26
#define PGT_pae_xen_l2      (1U<<_PGT_pae_xen_l2)

but (exp type = 0x70000000) & (PGT_type_mask|PGT_pae_xen_l2) =
0x60000000

So the exp type look strange.
#define _PGT_pinned         28
#define PGT_pinned          (1U<<_PGT_pinned)

> > 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
> 
> Um, it actually is from _etext -> __init_end + HPAGE_SIZE.
> 
> instrumenting the code a bit shows that setting of RW+NX from _etext
> throgh __init_end works just fine. It just when you start at the PFN
> _past_ the __init_end it dies. Any ideas? 
> 
Does this happen if you add ". = ALIGN(HPAGE_SIZE);" before bss section
in arch/x86/kernel/vmlinux.lds.S ?

Does it happen if you call set_pages_rw instead of set_pages_nx in
mark_nxdata_nx ?


What's the output of kernel_page_tables debugfs ?


Matthieu
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