Re: 4.4-rc0: 5 W+X pages found

[Kees Cook]

On Mon, Nov 23, 2015 at 6:37 AM, Mihai DonÈu <mihai.dontu@xxxxxxxxx> wrote:
> On Sun, 15 Nov 2015 08:00:22 +0100 Pavel Machek wrote:
>> Kernel complains:
>>
>> [    5.256044] ------------[ cut here ]------------
>> [    5.259267] WARNING: CPU: 0 PID: 1 at
>> arch/x86/mm/dump_pagetables.c:225 note_page+0x5ec/0x790()
>> [    5.262668] x86/mm: Found insecure W+X mapping at address
>> ffe69000/0xffe69000
>> [    5.267109] Modules linked in:
>> [    5.271403] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.3.0+ #122
>> [    5.275679] Hardware name: LENOVO 17097HU/17097HU, BIOS 7BETD8WW
>> (2.19 ) 03/31/2011
>> [    5.279957]  00000000 00000000 f5cffeac c42b9f18 f5cffed8 f5cffec8
>> c404062b 000000e1
>> [    5.284387]  c403ca9c f5cfff50 00000163 00000000 f5cffee0 c4040686
>> 00000009 f5cffed8
>> [    5.288815]  c4d268ac f5cffef4 f5cfff1c c403ca9c c4d1f494 000000e1
>> c4d268ac ffe69000
>> [    5.293314] Call Trace:
>> [    5.297602]  [<c42b9f18>] dump_stack+0x41/0x59
>> [    5.301864]  [<c404062b>] warn_slowpath_common+0x6b/0xa0
>> [    5.306054]  [<c403ca9c>] ? note_page+0x5ec/0x790
>> [    5.310209]  [<c4040686>] warn_slowpath_fmt+0x26/0x30
>> [    5.314358]  [<c403ca9c>] note_page+0x5ec/0x790
>> [    5.318440]  [<c403cd8f>] ptdump_walk_pgd_level_core+0x14f/0x230
>> [    5.322578]  [<c403ce91>] ptdump_walk_pgd_level_checkwx+0x11/0x20
>> [    5.326632]  [<c4034ead>] mark_rodata_ro+0xcd/0xf0
>> [    5.330625]  [<c4a4aab7>] kernel_init+0x17/0xc0
>> [    5.334585]  [<c4a511c9>] ret_from_kernel_thread+0x21/0x38
>> [    5.338585]  [<c4a4aaa0>] ? rest_init+0xa0/0xa0
>> [    5.342583] ---[ end trace bc9ac0874ad9a058 ]---
>> [    5.346630] x86/mm: Checked W+X mappings: FAILED, 5 W+X pages
>> found.
>>
>> ...I'm not quite sure why it does backtrace, or how to debug this
>> one...
>
> That is a modest number.
>
> [    2.493559] ------------[ cut here ]------------
> [    2.493563] WARNING: CPU: 2 PID: 1 at arch/x86/mm/dump_pagetables.c:225 note_page+0x5e1/0x780()
> [    2.493565] x86/mm: Found insecure W+X mapping at address ffff88000009d000/0xffff88000009d000
> [    2.493565] Modules linked in:
> [    2.493568] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.4.0-rc2 #2
> [    2.493569] Hardware name: Dell Inc. Latitude E7440/07F3F4, BIOS A15 05/19/2015
> [    2.493570]  0000000000000000 00000000c03551f4 ffff88040c7cbd48 ffffffffaa54851c
> [    2.493572]  ffff88040c7cbd90 ffff88040c7cbd80 ffffffffaa13a662 ffff88040c7cbe90
> [    2.493573]  8000000000000163 0000000000000004 0000000000000000 0000000000000000
> [    2.493575] Call Trace:
> [    2.493579]  [<ffffffffaa54851c>] dump_stack+0x4e/0x82
> [    2.493582]  [<ffffffffaa13a662>] warn_slowpath_common+0x82/0xc0
> [    2.493583]  [<ffffffffaa13a6fc>] warn_slowpath_fmt+0x5c/0x80
> [    2.493585]  [<ffffffffaa0a3f61>] note_page+0x5e1/0x780
> [    2.493587]  [<ffffffffaa0a43c4>] ptdump_walk_pgd_level_core+0x2c4/0x3f0
> [    2.493588]  [<ffffffffaa0a4527>] ptdump_walk_pgd_level_checkwx+0x17/0x20
> [    2.493591]  [<ffffffffaa09ad0f>] mark_rodata_ro+0xef/0x100
> [    2.493594]  [<ffffffffaae75d00>] ? rest_init+0x90/0x90
> [    2.493595]  [<ffffffffaae75d1d>] kernel_init+0x1d/0xe0
> [    2.493596]  [<ffffffffaae7c52f>] ret_from_fork+0x3f/0x70
> [    2.493598]  [<ffffffffaae75d00>] ? rest_init+0x90/0x90
> [    2.493599] ---[ end trace e2aec56d15b94609 ]---
> [    2.498994] x86/mm: Checked W+X mappings: FAILED, 104640 W+X pages found.
>
> All the while I have:
>
> $ zgrep NX /proc/config.gz
> CONFIG_DEBUG_SET_MODULE_RONX=y
>
> I added to CC the people involved in pushing this feature to mainline,
> maybe they can point me to a possible cause.

If you enable CONFIG_X86_PTDUMP, see if you can find out what exists
in /sys/kernel/debug/kernel_page_tables at ffff88000009d000 ?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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