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Date:	Thu, 05 Nov 2015 10:13:56 +0100
From:	Sander Eikelenboom <linux@...elenboom.it>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Stephen Smalley <sds@...ho.nsa.gov>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xen.org, mingo.kernel.org@...il.com
Subject: Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with
 CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core

On 2015-11-05 00:13, Boris Ostrovsky wrote:
> On 11/04/2015 03:02 PM, Sander Eikelenboom wrote:
>> On 2015-11-04 19:47, Stephen Smalley wrote:
>>> On 11/04/2015 01:28 PM, Sander Eikelenboom wrote:
>>>> On 2015-11-04 16:52, Stephen Smalley wrote:
>>>>> On 11/04/2015 06:55 AM, Sander Eikelenboom wrote:
>>>>>> Hi All,
>>>>>> 
>>>>>> I just tried to boot with the current linus mergewindow tree under 
>>>>>> Xen.
>>>>>> It fails with a kernel panic at boot with the new 
>>>>>> "CONFIG_DEBUG_WX"
>>>>>> option enabled.
>>>>>> Disabling it makes the kernel boot fine.
>>>>>> 
>>>>>> The splat:
>>>>>> [   18.424241] Freeing unused kernel memory: 1104K 
>>>>>> (ffffffff822fc000 -
>>>>>> ffffffff82410000)
>>>>>> [   18.430314] Write protecting the kernel read-only data: 18432k
>>>>>> [   18.441054] Freeing unused kernel memory: 1144K 
>>>>>> (ffff880001ae2000 -
>>>>>> ffff880001c00000)
>>>>>> [   18.447966] Freeing unused kernel memory: 1560K 
>>>>>> (ffff88000207a000 -
>>>>>> ffff880002200000)
>>>>>> [   18.453947] BUG: unable to handle kernel paging request at
>>>>>> ffff88055c883000
>>>>>> [   18.459943] IP: [<ffffffff8105af8e>]
>>>>>> ptdump_walk_pgd_level_core+0x20e/0x440
>>>>>> [   18.465847] PGD 2212067 PUD 0
>>>>>> [   18.471564] Oops: 0000 [#1] SMP
>>>>>> [   18.477248] Modules linked in:
>>>>>> [   18.482918] CPU: 2 PID: 1 Comm: swapper/0 Not tainted
>>>>>> 4.3.0-mw-20151104-linus-doflr+ #1
>>>>>> [   18.488804] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640)  , 
>>>>>> BIOS
>>>>>> V1.8B1 09/13/2010
>>>>>> [   18.494778] task: ffff880059b90000 ti: ffff880059b98000 
>>>>>> task.ti:
>>>>>> ffff880059b98000
>>>>>> [   18.500852] RIP: e030:[<ffffffff8105af8e>] [<ffffffff8105af8e>]
>>>>>> ptdump_walk_pgd_level_core+0x20e/0x440
>>>>>> [   18.507102] RSP: e02b:ffff880059b9be48  EFLAGS: 00010296
>>>>>> [   18.513351] RAX: ffff88055c883000 RBX: ffffffff81ae2000 RCX:
>>>>>> ffff880000000000
>>>>>> [   18.519733] RDX: 0000000000000067 RSI: ffff880059b9be98 RDI:
>>>>>> ffff880000001000
>>>>>> [   18.526129] RBP: ffff880059b9bf00 R08: 0000000000000000 R09:
>>>>>> 0000000000000000
>>>>>> [   18.532522] R10: ffff88005fd0e790 R11: 0000000000000001 R12:
>>>>>> ffff880080000000
>>>>>> [   18.538891] R13: ffffc00000000fff R14: ffff880059b9be98 R15:
>>>>>> 0000000000000000
>>>>>> [   18.545247] FS:  0000000000000000(0000) 
>>>>>> GS:ffff88005f680000(0000)
>>>>>> knlGS:0000000000000000
>>>>>> [   18.551708] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
>>>>>> [   18.558153] CR2: ffff88055c883000 CR3: 0000000002211000 CR4:
>>>>>> 0000000000000660
>>>>>> [   18.564686] Stack:
>>>>>> [   18.571106]  0000000159b9be50 ffffffff82211000 ffff88055c884000
>>>>>> 0000000000000800
>>>>>> [   18.577704]  0000800000000000 ffff88055c883000 0000000000000007
>>>>>> ffff88005fd0e790
>>>>>> [   18.584291]  ffff880059b9bed8 ffffffff81156ace 0000000000000001
>>>>>> 0000000000000000
>>>>>> [   18.590916] Call Trace:
>>>>>> [   18.597458]  [<ffffffff81156ace>] ? 
>>>>>> free_reserved_area+0x11e/0x120
>>>>>> [   18.604180]  [<ffffffff8105b1e2>]
>>>>>> ptdump_walk_pgd_level_checkwx+0x12/0x20
>>>>>> [   18.611014]  [<ffffffff810515b9>] mark_rodata_ro+0xe9/0xf0
>>>>>> [   18.617819]  [<ffffffff81ad3380>] ? rest_init+0x80/0x80
>>>>>> [   18.624512]  [<ffffffff81ad3398>] kernel_init+0x18/0xe0
>>>>>> [   18.631095]  [<ffffffff81adadcf>] ret_from_fork+0x3f/0x70
>>>>>> [   18.637650]  [<ffffffff81ad3380>] ? rest_init+0x80/0x80
>>>>>> [   18.644178] Code: 70 ff ff ff 48 3b 85 58 ff ff ff 0f 84 c0 fe 
>>>>>> ff ff
>>>>>> 48 8b 85 68 ff ff ff 48 c1 e0 10 48 c1 f8 10 48 89 45 b0 48 8b 85 
>>>>>> 70 ff
>>>>>> ff ff <48> 8b 38 48 85 ff 0f 85 4e ff ff ff b9 02 00 00 00 31 d2 
>>>>>> 4c 89
>>>>>> [   18.658246] RIP  [<ffffffff8105af8e>]
>>>>>> ptdump_walk_pgd_level_core+0x20e/0x440
>>>>>> [   18.665211]  RSP <ffff880059b9be48>
>>>>>> [   18.672073] CR2: ffff88055c883000
>>>>>> [   18.678852] ---[ end trace d84e34461c40637a ]---
>>>>>> [   18.685641] Kernel panic - not syncing: Attempted to kill init!
>>>>>> exitcode=0x00000009
>>>>>> [   18.685641]
>>>>>> [   18.699520] Kernel Offset: disable
>>>>>> 
>>>>> 
>>>>> What's your .config?  Does cat /sys/kernel/debug/kernel_page_tables
>>>>> produce a similar fault even with CONFIG_DEBUG_WX=n?
>>>> 
>>>> .config is attached
>>>> 
>>>> Hmm that sysfs file doesn't seem to exist then:
>>>> # cat /sys/kernel/debug/kernel_page_tables
>>>> cat: /sys/kernel/debug/kernel_page_tables: No such file or directory
>>> 
>>> Needs CONFIG_X86_PTDUMP=y.
>>> Also assumes you have debugfs mounted there.
>> 
>> Recompiled, and the result is that it also blows up:
>> 
> 
> Can you try this:
> 
> 
> diff --git a/arch/x86/mm/dump_pagetables.c 
> b/arch/x86/mm/dump_pagetables.c
> index 1bf417e..b534216 100644
> --- a/arch/x86/mm/dump_pagetables.c
> +++ b/arch/x86/mm/dump_pagetables.c
> @@ -362,8 +362,13 @@ static void ptdump_walk_pgd_level_core(struct
> seq_file *m, pgd_t *pgd,
>                         bool checkwx)
>  {
>  #ifdef CONFIG_X86_64
> +/* ffff800000000000 - ffff87ffffffffff is reserved for hypervisor */
> +#define is_hypervisor_range(idx)  (paravirt_enabled() && \
> +                  ((idx >= pgd_index(__PAGE_OFFSET) - 16) && \
> +                   (idx < pgd_index(__PAGE_OFFSET))))
>      pgd_t *start = (pgd_t *) &init_level4_pgt;
>  #else
> +#define is_hypervisor_range(idx)   0
>      pgd_t *start = swapper_pg_dir;
>  #endif
>      pgprotval_t prot;
> @@ -381,7 +386,7 @@ static void ptdump_walk_pgd_level_core(struct
> seq_file *m, pgd_t *pgd,
> 
>      for (i = 0; i < PTRS_PER_PGD; i++) {
>          st.current_address = normalize_addr(i * PGD_LEVEL_MULT);
> -        if (!pgd_none(*start)) {
> +        if (!pgd_none(*start) && !is_hypervisor_range(i)) {
>              if (pgd_large(*start) || !pgd_present(*start)) {
>                  prot = pgd_flags(*start);
>                  note_page(m, &st, __pgprot(prot), 1);

Hi Boris,

Thank for your patch !
It makes "cat /sys/kernel/debug/kernel_page_tables" work and
prevents a kernel with CONFIG_DEBUG_WX=y from crashing at boot.

It now does give a warning about an insecure W+X mapping, so 
CONFIG_DEBUG_WX=y
seems to be working. No idea how to interpret it though (and if it's a 
legit
warning).

--
Sander

[   19.034706] Freeing unused kernel memory: 1104K (ffffffff822fc000 - 
ffffffff82410000)
[   19.041339] Write protecting the kernel read-only data: 18432k
[   19.052596] Freeing unused kernel memory: 1144K (ffff880001ae2000 - 
ffff880001c00000)
[   19.060285] Freeing unused kernel memory: 1560K (ffff88000207a000 - 
ffff880002200000)
[   19.067079] ------------[ cut here ]------------
[   19.073931] WARNING: CPU: 5 PID: 1 at 
arch/x86/mm/dump_pagetables.c:225 note_page+0x619/0x7e0()
[   19.081039] x86/mm: Found insecure W+X mapping at address 
ffff880000000000/0xffff880000000000
[   19.088293] Modules linked in:
[   19.095477] CPU: 5 PID: 1 Comm: swapper/0 Not tainted 
4.3.0-mw-20151104-linus-doflr-withdebugwx-noptdump-ptdumppatch+ #1
[   19.102971] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640)  , BIOS 
V1.8B1 09/13/2010
[   19.110594]  ffffffff81f381d3 ffff880059b9bd60 ffffffff81428992 
ffff880059b9bda8
[   19.118239]  ffff880059b9bd98 ffffffff810ca7ec ffff8800025d8010 
0000000000000004
[   19.125870]  0010000000000067 ffff880059b9be98 0000000000000000 
ffff880059b9bdf8
[   19.133526] Call Trace:
[   19.141212]  [<ffffffff81428992>] dump_stack+0x44/0x62
[   19.148983]  [<ffffffff810ca7ec>] warn_slowpath_common+0x7c/0xb0
[   19.156877]  [<ffffffff810ca867>] warn_slowpath_fmt+0x47/0x50
[   19.164813]  [<ffffffff8105abb9>] note_page+0x619/0x7e0
[   19.172656]  [<ffffffff8105b135>] 
ptdump_walk_pgd_level_core+0x3b5/0x450
[   19.180583]  [<ffffffff8105b1f2>] 
ptdump_walk_pgd_level_checkwx+0x12/0x20
[   19.188427]  [<ffffffff810515b9>] mark_rodata_ro+0xe9/0xf0
[   19.196221]  [<ffffffff81ad2f70>] ? rest_init+0x80/0x80
[   19.204012]  [<ffffffff81ad2f88>] kernel_init+0x18/0xe0
[   19.211732]  [<ffffffff81ada98f>] ret_from_fork+0x3f/0x70
[   19.219467]  [<ffffffff81ad2f70>] ? rest_init+0x80/0x80
[   19.227145] ---[ end trace 5fc0f4297b911570 ]---
[   19.244382] x86/mm: Checked W+X mappings: FAILED, 4602 W+X pages 
found.

--
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