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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/04/2015 01:47 PM, 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.

I don't think this code (including original page tables dump) can work 
on Xen PV guests: we are walking full PGD, including the hypervisor hole 
(entries 256-272).
-boris


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