Re: next-20170515: WARNING: CPU: 0 PID: 1 at arch/x86/mm/dump_pagetables.c:236 note_page+0x630/0x7e0

From: Luis R. Rodriguez
Date: Thu May 18 2017 - 23:08:17 EST

On Fri, May 19, 2017 at 02:44:14AM +0200, Luis R. Rodriguez wrote:
> On Wed, May 17, 2017 at 10:53:06AM -0700, Kees Cook wrote:
> > On Wed, May 17, 2017 at 9:40 AM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> wrote:
> > > Yes, but I had killed that boot session again, so upon my next boot
> > > I had a different layout, the ASLR gap was much larger:
> > >
> > > ---[ Modules ]---
> > > 0xffffffffc0000000-0xffffffffc01b0000 1728K pte
> > > 0xffffffffc01b0000-0xffffffffc01b1000 4K RW GLB x pte
> > > 0xffffffffc01b1000-0xffffffffc01b2000 4K pte
> > > 0xffffffffc01b2000-0xffffffffc01c6000 80K ro GLB x pte
> > > 0xffffffffc01c6000-0xffffffffc01cc000 24K ro GLB NX pte
> > > 0xffffffffc01cc000-0xffffffffc01d5000 36K RW GLB NX pte
> > >
> > > As you can guess if we follow similar pattern the RW hole is the one this boot
> > > warned about:
> > >
> > > [ 1.450483] x86/mm: Found insecure W+X mapping at address ffffffffc01b0000/0xffffffffc01b0000
> > > [ 1.451280] ------------[ cut here ]------------
> > > [ 1.451721] WARNING: CPU: 1 PID: 1 at arch/x86/mm/dump_pagetables.c:236 note_page+0x630/0x7e0
> > > [ 1.452499] Modules linked in:
> > > [ 1.452791] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.12.0-rc1-next-20170515+ #145
> > >
> > > I checked and indeed 0xffffffffc01b2000 is part of a module, it was not the first one
> > > on the /proc/modules list but then again /proc/modules does not seem to have a specific
> > > order other than perhaps being pegged into a linked list of modules once they go live,
> > > and it seems its typically output backwards from when that happened, sorting that
> > > by address we get:
> >
> > Right, sorry, I'd expect it at the bottom of the list in
> > /proc/modules, but that's fine, it's there.
> >
> > >
> > > root@piggy:~# cat /proc/modules | sort -k 6 | head -3
> > > e1000 143360 0 - Live 0xffffffffc01b2000 (E)
> > > mbcache 16384 1 ext4, Live 0xffffffffc01d6000 (E)
> > > scsi_mod 217088 4 sg,sr_mod,sd_mod,libata, Live 0xffffffffc01df000 (E)
> > >
> > > And this then seems to be the first module loaded:
> > >
> > > e1000 143360 0 - Live 0xffffffffc01b2000 (E)
> > >
> > > The output of dmesg seems to confirm this as per the list of modules sorted
> > > as per above.
> > >
> > >> Something touched the module gap and left is RW+x...
> > >
> > > Lemme try booting with e1000 renamed to e1000.ko.ignore and see how that goes.
> >
> > Is it possible a module got loaded before e1000 and then unloaded?
> > That seems odd, but maybe unload isn't cleaning up?
> >
> > >> Are you able to bisect this?
> > >
> > > This issue has been present for a while so since I recall this I might be
> > > able to reduce the number of needed target kernels to bisect. Lemme tinker
> > > a bit and if no clear culprit comes up then will try bisect.
> >
> > Okay, thanks!
>
> Sorry to report that this issue is present since the feature's addition. So
> the issue is there since its addition and is still present today. *But* it
> may also be a configuration issue, given I have booted this guest *without*
> this issue ...
>
> So:
>
> git checkout -b WX e1a58320a38dfa72be48a0f1a3a92273663ba6db
>
> That boots with the warning. To help debug further I've minimized my modules
> to only a few: scsi_mod, e1000, libata.
>
> I suspect at this point this is not the fault of a particular module but
> instead just an accounting semantic (>= or <= on an edge) but let's see.
>
> I now boot on 4.3.0-rc3 on commit (e1a58320a38df ("x86/mm: Warn on W^X
> mappings") and I with:
>
> [ 0.949435] ------------[ cut here ]------------
> [ 0.949992] WARNING: CPU: 2 PID: 1 at arch/x86/mm/dump_pagetables.c:225 note_page+0x635/0x7e0()
> [ 0.950996] x86/mm: Found insecure W+X mapping at address ffffffffc0000000/0xffffffffc0000000
> [ 0.951814] Modules linked in:
> [ 0.952123] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.3.0-rc3-FINAL-TEST-WITH-WX-NOFLOPPY+ #365
> [ 0.952929] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.10.2-0-g5f4c7b1-prebuilt.qemu-project.org 04/01/2014
> [ 0.954033] 0000000000000000 000000001f722925 ffff88013a5d7d40 ffffffff812ff335
> [ 0.954742] ffff88013a5d7d88 ffff88013a5d7d78 ffffffff81079be2 ffff88013a5d7e90
> [ 0.955522] 0000000000000000 0000000000000004 0000000000000000 0000000000000000
> [ 0.956256] Call Trace:
> [ 0.956496] [<ffffffff812ff335>] dump_stack+0x44/0x5f
> [ 0.956953] [<ffffffff81079be2>] warn_slowpath_common+0x82/0xc0
> [ 0.957519] [<ffffffff81079c7c>] warn_slowpath_fmt+0x5c/0x80
> [ 0.958066] [<ffffffff8106c155>] note_page+0x635/0x7e0
> [ 0.958595] [<ffffffff8106c5eb>] ptdump_walk_pgd_level_core+0x2eb/0x410
> [ 0.959219] [<ffffffff8106c7b7>] ptdump_walk_pgd_level_checkwx+0x17/0x20
> [ 0.959856] [<ffffffff8106260d>] mark_rodata_ro+0xed/0x100
> [ 0.960372] [<ffffffff815aa7d0>] ? rest_init+0x80/0x80
> [ 0.960869] [<ffffffff815aa7ed>] kernel_init+0x1d/0xe0
> [ 0.961358] [<ffffffff815b798f>] ret_from_fork+0x3f/0x70
> [ 0.961900] [<ffffffff815aa7d0>] ? rest_init+0x80/0x80
> [ 0.962389] ---[ end trace 6125ebcb24c9e3d0 ]---
> [ 0.962822] x86/mm: Checked W+X mappings: FAILED, 1 W+X pages found.
>
>
> ---[ High Kernel Mapping ]---
> 0xffffffff80000000-0xffffffff81000000 16M pmd
> 0xffffffff81000000-0xffffffff81600000 6M ro PSE GLB x pmd
> 0xffffffff81600000-0xffffffff81a00000 4M ro PSE GLB NX pmd
> 0xffffffff81a00000-0xffffffff81c00000 2M RW GLB NX pte
> 0xffffffff81c00000-0xffffffff82200000 6M RW PSE GLB NX pmd
> 0xffffffff82200000-0xffffffff82400000 2M RW GLB NX pte
> 0xffffffff82400000-0xffffffffc0000000 988M pmd
> ---[ Modules ]---
> 0xffffffffc0000000-0xffffffffc0001000 4K RW GLB x pte
> 0xffffffffc0001000-0xffffffffc0002000 4K pte
> 0xffffffffc0002000-0xffffffffc0039000 220K RW GLB x pte
>
> root@piggy:~# cat /proc/modules | sort -k 6 | head -3
> scsi_mod 221979 4 sg,sd_mod,sr_mod,libata, Live 0xffffffffc0002000 (E)
> e1000 127757 0 - Live 0xffffffffc004d000 (E)
> libata 229931 2 ata_generic,ata_piix, Live 0xffffffffc0076000 (E)
>
> So that 4K RW seems suspect of getting used for allocation purpose on edge
> for a particular reason and it also happens to be on the edge of the high
> kernel mapping. Could it be the boundary semantic issue ?
>
> For instance can it be that since 0xffffffffc0002000 is given to the first
> module by the allocator, scsi_mod, and since that address is *technically*
> part of two boundaries we get a splat ?
>
> 0xffffffffc0001000-0xffffffffc0002000 4K pte
> 0xffffffffc0002000-0xffffffffc0039000 220K RW GLB x pte

Note on the latest linux-next and on the commit that introduced this the config
and kernel yields only *one* page:

x86/mm: Checked W+X mappings: FAILED, 1 W+X pages found.

I believe this is more indications my suspicion might be right.

Luis