Re: Linux 4.4 MW: Boot under Xen fails with CONFIG_DEBUG_WX enabled: RIP: ptdump_walk_pgd_level_core

From: Sander Eikelenboom
Date: Wed Nov 04 2015 - 15:03:51 EST

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:
[ 902.389247] BUG: unable to handle kernel paging request at ffff88055c883000
[ 902.402749] IP: [<ffffffff8105af7e>] ptdump_walk_pgd_level_core+0x20e/0x440
[ 902.416261] PGD 2212067 PUD 0
[ 902.427768] Oops: 0000 [#1] SMP
[ 902.438137] Modules linked in:
[ 902.448299] CPU: 2 PID: 21951 Comm: cat Not tainted 4.3.0-mw-20151104-linus-doflr-nodebugwx-withptdump+ #1
[ 902.458581] Hardware name: MSI MS-7640/890FXA-GD70 (MS-7640) , BIOS V1.8B1 09/13/2010
[ 902.468850] task: ffff88004b49e300 ti: ffff88005928c000 task.ti: ffff88005928c000
[ 902.479133] RIP: e030:[<ffffffff8105af7e>] [<ffffffff8105af7e>] ptdump_walk_pgd_level_core+0x20e/0x440
[ 902.489536] RSP: e02b:ffff88005928fd20 EFLAGS: 00010296
[ 902.499692] RAX: ffff88055c883000 RBX: 0000000000000000 RCX: ffff880000000000
[ 902.509755] RDX: 0000000000000067 RSI: ffff88005928fd70 RDI: ffff880000001000
[ 902.519680] RBP: ffff88005928fdd8 R08: 0000000000001000 R09: 000000000000ffff
[ 902.529555] R10: 0000000000000000 R11: 0000000000000246 R12: ffff88005928ff20
[ 902.539349] R13: ffffc00000000fff R14: ffff88005928fd70 R15: ffff880033c773c0
[ 902.549081] FS: 00007f56b07d4700(0000) GS:ffff88005f680000(0000) knlGS:0000000000000000
[ 902.558690] CS: e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 902.568111] CR2: ffff88055c883000 CR3: 000000004563f000 CR4: 0000000000000660
[ 902.577508] Stack:
[ 902.586741] 00000000810ffd21 ffffffff82211000 ffff88055c884000 0000000000000800
[ 902.596090] 0000800000000000 ffff88055c883000 00007f56b0629000 0000000000000001
[ 902.605395] ffff88005928ff20 ffffffff81ad88ba 0000000000000001 0000000000000000
[ 902.614452] Call Trace:
[ 902.623254] [<ffffffff81ad88ba>] ? __mutex_lock_slowpath+0x27a/0x3a0
[ 902.632067] [<ffffffff8105b1bd>] ptdump_show+0xd/0x20
[ 902.640767] [<ffffffff811c857b>] seq_read+0xcb/0x380
[ 902.649379] [<ffffffff811a8273>] __vfs_read+0x23/0xd0
[ 902.657909] [<ffffffff81167cb5>] ? vm_mmap_pgoff+0x75/0xa0
[ 902.666195] [<ffffffff811a8a01>] vfs_read+0x81/0x130
[ 902.674248] [<ffffffff811a96f4>] SyS_read+0x44/0xb0
[ 902.682220] [<ffffffff81ada6ae>] entry_SYSCALL_64_fastpath+0x12/0x71
[ 902.690182] 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
[ 902.706831] RIP [<ffffffff8105af7e>] ptdump_walk_pgd_level_core+0x20e/0x440
[ 902.715008] RSP <ffff88005928fd20>
[ 902.722955] CR2: ffff88055c883000
[ 902.730630] ---[ end trace 5d94ee3d1b42814b ]---

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