Re: NULL pointer dereference in xenbus_thread->...

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Aug 28, 2023 at 11:50:36PM +0200, Marek Marczykowski-Górecki wrote:
> Hi,
> 
> I've noticed in Qubes's CI failure like this:
> 
> [  871.271292] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [  871.275290] #PF: supervisor read access in kernel mode
> [  871.277282] #PF: error_code(0x0000) - not-present page
> [  871.279182] PGD 106fdb067 P4D 106fdb067 PUD 106fdc067 PMD 0 
> [  871.281071] Oops: 0000 [#1] PREEMPT SMP NOPTI
> [  871.282698] CPU: 1 PID: 28 Comm: xenbus Not tainted 
> 6.1.43-1.qubes.fc37.x86_64 #1
> [  871.285222] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 
> rel-1.16.0-0-gd239552-rebuilt.opensuse.org 04/01/2014
> [  871.288883] RIP: e030:__wake_up_common+0x4c/0x180
> [  871.292838] Code: 24 0c 89 4c 24 08 4d 85 c9 74 0a 41 f6 01 04 0f 85 a3 00 
> 00 00 48 8b 43 08 4c 8d 40 e8 48 83 c3 08 49 8d 40 18 48 39 c3 74 5b <49> 8b 
> 40 18 31 ed 4c 8d 70 e8 45 8b 28 41 f6 c5 04 75 5f 49 8b 40
> [  871.299776] RSP: e02b:ffffc900400f7e10 EFLAGS: 00010082
> [  871.301656] RAX: 0000000000000000 RBX: ffff88810541ce98 RCX: 
> 0000000000000000
> [  871.304255] RDX: 0000000000000001 RSI: 0000000000000003 RDI: 
> ffff88810541ce90
> [  871.306714] RBP: ffffc900400f0280 R08: ffffffffffffffe8 R09: 
> ffffc900400f7e68
> [  871.309937] R10: 0000000000007ff0 R11: ffff888100ad3000 R12: 
> ffffc900400f7e68
> [  871.312326] R13: 0000000000000000 R14: 0000000000000000 R15: 
> 0000000000000000
> [  871.314647] FS:  0000000000000000(0000) GS:ffff88813ff00000(0000) 
> knlGS:0000000000000000
> [  871.317677] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  871.319644] CR2: 0000000000000000 CR3: 00000001067fe000 CR4: 
> 0000000000040660
> [  871.321973] Call Trace:
> [  871.322782]  <TASK>
> [  871.323494]  ? show_trace_log_lvl+0x1d3/0x2ef
> [  871.324901]  ? show_trace_log_lvl+0x1d3/0x2ef
> [  871.326310]  ? show_trace_log_lvl+0x1d3/0x2ef
> [  871.327721]  ? __wake_up_common_lock+0x82/0xd0
> [  871.329147]  ? __die_body.cold+0x8/0xd
> [  871.330378]  ? page_fault_oops+0x163/0x1a0
> [  871.331691]  ? exc_page_fault+0x70/0x170
> [  871.332946]  ? asm_exc_page_fault+0x22/0x30
> [  871.334454]  ? __wake_up_common+0x4c/0x180
> [  871.335777]  __wake_up_common_lock+0x82/0xd0
> [  871.337183]  ? process_writes+0x240/0x240
> [  871.338461]  process_msg+0x18e/0x2f0
> [  871.339627]  xenbus_thread+0x165/0x1c0
> [  871.340830]  ? cpuusage_read+0x10/0x10
> [  871.342032]  kthread+0xe9/0x110
> [  871.343317]  ? kthread_complete_and_exit+0x20/0x20
> [  871.345020]  ret_from_fork+0x22/0x30
> [  871.346239]  </TASK>
> [  871.347060] Modules linked in: snd_hda_codec_generic ledtrig_audio 
> snd_hda_intel snd_intel_dspcfg snd_intel_sdw_acpi snd_hda_codec snd_hda_core 
> snd_hwdep snd_seq snd_seq_device joydev snd_pcm intel_rapl_msr ppdev 
> intel_rapl_common snd_timer pcspkr e1000e snd soundcore i2c_piix4 parport_pc 
> parport loop fuse xenfs dm_crypt crct10dif_pclmul crc32_pclmul crc32c_intel 
> polyval_clmulni polyval_generic floppy ghash_clmulni_intel sha512_ssse3 
> serio_raw virtio_scsi virtio_console bochs xhci_pci xhci_pci_renesas xhci_hcd 
> qemu_fw_cfg drm_vram_helper drm_ttm_helper ttm ata_generic pata_acpi 
> xen_privcmd xen_pciback xen_blkback xen_gntalloc xen_gntdev xen_evtchn 
> scsi_dh_rdac scsi_dh_emc scsi_dh_alua uinput dm_multipath
> [  871.368892] CR2: 0000000000000000
> [  871.370160] ---[ end trace 0000000000000000 ]---
> [  871.371719] RIP: e030:__wake_up_common+0x4c/0x180
> [  871.373273] Code: 24 0c 89 4c 24 08 4d 85 c9 74 0a 41 f6 01 04 0f 85 a3 00 
> 00 00 48 8b 43 08 4c 8d 40 e8 48 83 c3 08 49 8d 40 18 48 39 c3 74 5b <49> 8b 
> 40 18 31 ed 4c 8d 70 e8 45 8b 28 41 f6 c5 04 75 5f 49 8b 40
> [  871.379866] RSP: e02b:ffffc900400f7e10 EFLAGS: 00010082
> [  871.381689] RAX: 0000000000000000 RBX: ffff88810541ce98 RCX: 
> 0000000000000000
> [  871.383971] RDX: 0000000000000001 RSI: 0000000000000003 RDI: 
> ffff88810541ce90
> [  871.386235] RBP: ffffc900400f0280 R08: ffffffffffffffe8 R09: 
> ffffc900400f7e68
> [  871.388521] R10: 0000000000007ff0 R11: ffff888100ad3000 R12: 
> ffffc900400f7e68
> [  871.390789] R13: 0000000000000000 R14: 0000000000000000 R15: 
> 0000000000000000
> [  871.393101] FS:  0000000000000000(0000) GS:ffff88813ff00000(0000) 
> knlGS:0000000000000000
> [  871.395671] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  871.397863] CR2: 0000000000000000 CR3: 00000001067fe000 CR4: 
> 0000000000040660
> [  871.400441] Kernel panic - not syncing: Fatal exception
> [  871.402171] Kernel Offset: disabled
> (XEN) Hardware Dom0 crashed: rebooting machine in 5 seconds.
> 
> It isn't the first time I see similar crash, but I can't really
> reproduce it reliably. Restarted test usually passes.
> Note this is Xen nested in KVM, so it could very well be some oddity
> about nested virt, although looking at the stack trace, it's unlikely
> and more likely some race condition hit only on slower system.

Recently I've got the same crash on a real system in domU too. And also
on nested on newer kernel 6.1.57 (here it happened in dom0). So, this is
still an issue and affects not only nested case :/

> Unfortunately I don't have symbols for this kernel handy, but there is a
> single wake_up() call in process_writes(), so it shouldn't be an issue.
> 
> Any ideas?
> 
> Full log at 
> https://openqa.qubes-os.org/tests/80779/logfile?filename=serial0.txt

More links at https://github.com/QubesOS/qubes-issues/issues/8638,
including more recent stack trace.


-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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