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

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Tue, Apr 29, 2025 at 08:59:45PM -0400, Jason Andryuk wrote:
> Hi Marek,
> 
> On Wed, Apr 23, 2025 at 8:42 AM Marek Marczykowski-Górecki
> <marmarek@invisiblethingslab.com> wrote:
> >
> > On Sat, Jun 01, 2024 at 12:48:33AM +0200, Marek Marczykowski-Górecki wrote:
> > > On Tue, Mar 26, 2024 at 11:00:50AM +0000, Julien Grall wrote:
> > > > Hi Marek,
> > > >
> > > > +Juergen for visibility
> > > >
> > > > When sending a bug report, I would suggest to CC relevant people as
> > > > otherwise it can get lost (not may people monitors Xen devel if they are not
> > > > CCed).
> > > >
> > > > Cheers,
> > > >
> > > > On 25/03/2024 16:17, Marek Marczykowski-Górecki wrote:
> > > > > On Sun, Oct 22, 2023 at 04:14:30PM +0200, Marek Marczykowski-Górecki wrote:
> > > > > > 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.
> > > > >
> > > > > Happens on 6.1.75 too (new stack trace I've added to the issue above,
> > > > > but it's pretty similar).
> > >
> > > Recently I've got a report from another user about similar issue, on
> > > 6.6.29 this time. I also still encounter this issue once a month or so,
> > > but the user claims they get it much more often:
> > > https://github.com/QubesOS/qubes-issues/issues/8638#issuecomment-2135419896
> > > The extra conditions reported by the user are:
> > > - old AMD system (KGPE-D16 with Opteron 6282 SE) requiring
> > >   `spec-ctrl=ibpb-entry=no-pv` to remain usable
> > > - Whonix domU, which has a bunch of sysctl parameters changed, listed
> > >   at:
> > >   - https://github.com/Kicksecure/security-misc
> > >   - https://github.com/Kicksecure/security-misc/blob/master/usr/lib/sysctl.d/990-security-misc.conf
> > >   (unsure which are relevant, maybe `vm.swappiness=1`?)
> >
> > I've got some more report confirming it's still happening on Linux
> > 6.12.18. Is there anything I can do to help fixing this? Maybe ask users
> > to enable some extra logging?
> 
> Have you been able to capture a crash with debug symbols and run it
> through scripts/decode_stacktrace.sh?

This worked:

    BUG: kernel NULL pointer dereference, address: 0000000000000000
    #PF: supervisor read access in kernel mode
    #PF: error_code(0x0000) - not-present page
    PGD 0 P4D 0
    Oops: Oops: 0000 [#1] PREEMPT SMP NOPTI
    RIP: 0010:__wake_up_common (kernel/sched/wait.c:85) 
    Code: 0f 1f 44 00 00 41 57 41 56 41 55 41 54 55 53 48 89 fb 48 83 c3 08 48 83 ec 08 48 8b 47 08 89 54 24 04 48 39 c3 74 55 4d 89 c7 <4c> 8b 00 41 89 f5 41 89 ce 48 8d 78 e8 4d 8d 60 e8 eb 27 74 0c 83
    All code
    ========
       0:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)
       5:	41 57                	push   %r15
       7:	41 56                	push   %r14
       9:	41 55                	push   %r13
       b:	41 54                	push   %r12
       d:	55                   	push   %rbp
       e:	53                   	push   %rbx
       f:	48 89 fb             	mov    %rdi,%rbx
      12:	48 83 c3 08          	add    $0x8,%rbx
      16:	48 83 ec 08          	sub    $0x8,%rsp
      1a:	48 8b 47 08          	mov    0x8(%rdi),%rax
      1e:	89 54 24 04          	mov    %edx,0x4(%rsp)
      22:	48 39 c3             	cmp    %rax,%rbx
      25:	74 55                	je     0x7c
      27:	4d 89 c7             	mov    %r8,%r15
      2a:*	4c 8b 00             	mov    (%rax),%r8		<-- trapping instruction
      2d:	41 89 f5             	mov    %esi,%r13d
      30:	41 89 ce             	mov    %ecx,%r14d
      33:	48 8d 78 e8          	lea    -0x18(%rax),%rdi
      37:	4d 8d 60 e8          	lea    -0x18(%r8),%r12
      3b:	eb 27                	jmp    0x64
      3d:	74 0c                	je     0x4b
      3f:	83                   	.byte 0x83

    Code starting with the faulting instruction
    ===========================================
       0:	4c 8b 00             	mov    (%rax),%r8
       3:	41 89 f5             	mov    %esi,%r13d
       6:	41 89 ce             	mov    %ecx,%r14d
       9:	48 8d 78 e8          	lea    -0x18(%rax),%rdi
       d:	4d 8d 60 e8          	lea    -0x18(%r8),%r12
      11:	eb 27                	jmp    0x3a
      13:	74 0c                	je     0x21
      15:	83                   	.byte 0x83
    RSP: 0018:ffffc900009f7e40 EFLAGS: 00010082
    RAX: 0000000000000000 RBX: ffff8880109c0798 RCX: 0000000000000000
    RDX: 0000000000000001 RSI: 0000000000000003 RDI: ffff8880109c0790
    RBP: ffff8880109c0790 R08: 0000000000000000 R09: 0000000000000003
    R10: ffffc900009f7eb0 R11: ffffc9000003d000 R12: 0000000000000003
    R13: 0000000000000246 R14: 0000000000000000 R15: 0000000000000000
    FS:  0000000000000000(0000) GS:ffff888018500000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 0000000000000000 CR3: 00000000432a0000 CR4: 00000000000406f0
    Call Trace:
    <TASK>
    ? __die (arch/x86/kernel/dumpstack.c:421 arch/x86/kernel/dumpstack.c:434) 
    ? page_fault_oops (arch/x86/mm/fault.c:715) 
    ? exc_page_fault (arch/x86/include/asm/paravirt.h:693 arch/x86/mm/fault.c:1489 arch/x86/mm/fault.c:1539) 
    ? asm_exc_page_fault (arch/x86/include/asm/idtentry.h:623) 
    ? __wake_up_common (kernel/sched/wait.c:85) 
    ? __pfx_xenbus_thread (drivers/xen/xenbus/xenbus_comms.c:411) 
    __wake_up (include/linux/spinlock.h:406 kernel/sched/wait.c:108 kernel/sched/wait.c:127) 
    process_msg (drivers/xen/xenbus/xenbus_comms.c:311) 
    xenbus_thread (drivers/xen/xenbus/xenbus_comms.c:418) 
    ? __pfx_autoremove_wake_function (kernel/sched/wait.c:383) 
    kthread (kernel/kthread.c:389) 
    ? __pfx_kthread (kernel/kthread.c:342) 
    ret_from_fork (arch/x86/kernel/process.c:153) 
    ? __pfx_kthread (kernel/kthread.c:342) 
    ret_from_fork_asm (arch/x86/entry/entry_64.S:257) 
    </TASK>
    Modules linked in: snd_seq_dummy snd_hrtimer snd_seq snd_seq_device snd_timer snd soundcore xenfs nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink binfmt_misc crct10dif_pclmul crc32_pclmul crc32c_intel polyval_clmulni polyval_generic ghash_clmulni_intel sha512_ssse3 sha256_ssse3 xen_netfront sha1_ssse3 xen_privcmd xen_gntdev xen_gntalloc xen_blkback xen_evtchn loop fuse ip_tables overlay xen_blkfront
    CR2: 0000000000000000
    ---[ end trace 0000000000000000 ]---

> I'm curious what process_msg+0x18e/0x2f0 is.  process_writes() has a
> direct call to wake_up(), but process_msg() calling req->cb(req) may
> be xs_wake_up() which is a thin wrapper over wake_up().

So, it's req->cb(req).

> They make me wonder if req has been free()ed and at least partially
> zero-ed, but it still has wake_up() called.  The call stack here is
> reminiscent of the one here
> https://lore.kernel.org/xen-devel/Z_lJTyVipJJEpWg2@mail-itl/ and the
> unexpected value there is 0.
> 
> I haven't actually figured out a scenario where req would be kfree()ed
> early.  But the handling of kfree(req) being split between
> process_msg/writes() and xs_wait_for_reply() makes me a little uneasy.
> 
> Regards,
> Jason

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

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