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Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core

To: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stephen Smalley <sds@xxxxxxxxxxxxx>
From: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Date: Thu, 05 Nov 2015 10:13:56 +0100
Cc: x86@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, mingo.kernel.org@xxxxxxxxx, David Vrabel <david.vrabel@xxxxxxxxxx>
Delivery-date: Thu, 05 Nov 2015 09:15:39 +0000
List-id: Xen developer discussion <xen-devel.lists.xen.org>

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 underXen.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: ffff880059b98000task.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 feff ff48 8b 85 68 ff ff ff 48 c1 e0 10 48 c1 f8 10 48 89 45 b0 48 8b 8570 ffff ff <48> 8b 38 48 85 ff 0f 85 4e ff ff ff b9 02 00 00 00 31 d24c 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.cb/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, soCONFIG_DEBUG_WX=yseems to be working. No idea how to interpret it though (and if it's alegit

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 atarch/x86/mm/dump_pagetables.c:225 note_page+0x619/0x7e0()[ 19.081039] x86/mm: Found insecure W+X mapping at addressffff880000000000/0xffff880000000000

[   19.088293] Modules linked in:

[ 19.095477] CPU: 5 PID: 1 Comm: swapper/0 Not tainted4.3.0-mw-20151104-linus-doflr-withdebugwx-noptdump-ptdumppatch+ #1[ 19.102971] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640) , BIOSV1.8B1 09/13/2010[ 19.110594] ffffffff81f381d3 ffff880059b9bd60 ffffffff81428992ffff880059b9bda8[ 19.118239] ffff880059b9bd98 ffffffff810ca7ec ffff8800025d80100000000000000004[ 19.125870] 0010000000000067 ffff880059b9be98 0000000000000000ffff880059b9bdf8

[   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 pagesfound.



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Follow-Ups:
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Boris Ostrovsky

References:
- [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Sander Eikelenboom
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Stephen Smalley
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Sander Eikelenboom
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Stephen Smalley
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Sander Eikelenboom
- Re: [Xen-devel] Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core
  - From: Boris Ostrovsky

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