Message-ID: <25aff5ca-b5e1-4907-bd12-6571f8454146@linux.microsoft.com>
Date: Sat, 15 Nov 2025 14:33:54 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: Salvatore Bonaccorso <carnil@...ian.org>, 1120602@...s.debian.org
Cc: Peter Morrow <pdmorrow@...il.com>, Long Li <longli@...rosoft.com>,
 linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
 regressions@...ts.linux.dev, stable@...r.kernel.org,
 John Starks <jostarks@...rosoft.com>, Michael Kelley <mhklinux@...look.com>,
 Tianyu Lan <tiala@...rosoft.com>, "K. Y. Srinivasan" <kys@...rosoft.com>,
 Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: Bug#1120602: [REGRESSION 6.12.y] hyper-v: BUG: kernel NULL
 pointer dereference, address: 00000000000000a0: RIP:
 0010:hv_uio_channel_cb+0xd/0x20 [uio_hv_generic]



On 11/15/2025 3:14 AM, Salvatore Bonaccorso wrote:
> Hi,
> 
> On Fri, Nov 14, 2025 at 08:05:55PM +0530, Naman Jain wrote:
>>
>>
>> On 11/14/2025 5:19 PM, Peter Morrow wrote:
>>> Hi Naman,
>>>
>>> On Fri, 14 Nov 2025 at 06:03, Naman Jain <namjain@...ux.microsoft.com> wrote:
>>>>
>>>>
>>>>
>>>> On 11/13/2025 11:59 PM, Salvatore Bonaccorso wrote:
>>>>> Peter Morrow reported in Debian a regression, reported in
>>>>> https://bugs.debian.org/1120602 . The regression was seen after
>>>>> updating, to 6.12.57-1 in Debian, but details on the offending commit
>>>>> follows.
>>>>>
>>>>> His report was as follows:
>>>>>
>>>>>> Dear Maintainer,
>>>>>>
>>>>>> I'm seeing a kernel crash quite soon after boot on a debian trixie based
>>>>>> system running 6.12.57+deb13-amd64, unfortunately the kernel panics before
>>>>>> I can access the system to gather more information. Thus I'll provide details
>>>>>> of the system using a previously known good version. The panic is happening
>>>>>> 100% of the time unfortunately. I have access to the serial console however
>>>>>> so can enable any required verbose logging during boot if necessary.
>>>>>>
>>>>>> Crucially the crash is not seen with kernel version 6.12.41+deb13-amd64 with the
>>>>>> same userspace. We had pinned to that version until very recently to in order
>>>>>> to work around https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1109676
>>>>>>
>>>>>> I'm running a dpdk application here (VPP) on Azure, VM form factor is a
>>>>>> "Standard DS3 v2 (4 vcpus, 14 GiB memory)".
>>>>>>
>>>>>> The only relevant upstream commit in this area (as far as I can see) is:
>>>>>>
>>>>>> https://lore.kernel.org/linux-hyperv/1bb599ee-fe28-409d-b430-2fc086268936@linux.microsoft.com/
>>>>>>
>>>>>> The comment regarding avoiding races at start adds a bit more weight behind this
>>>>>> hunch, though it's only a hunch as I am most definitely nowhere near an expert
>>>>>> in this area.
>>>>>>
>>>>>> -- Package-specific info:
>>>>>>
>>>>>> [   19.625535] BUG: kernel NULL pointer dereference, address: 00000000000000a0
>>>>>> [   19.628874] #PF: supervisor read access in kernel mode
>>>>>> [   19.630841] #PF: error_code(0x0000) - not-present page
>>>>>> [   19.632788] PGD 0 P4D 0
>>>>>> [   19.633905] Oops: Oops: 0000 [#1] PREEMPT SMP PTI
>>>>>> [   19.635586] CPU: 3 UID: 0 PID: 0 Comm: swapper/3 Not tainted 6.12.57+deb13-amd64 #1  Debian 6.12.57-1
>>>>>> [   19.640216] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 09/28/2024
>>>>>> [   19.644514] RIP: 0010:hv_uio_channel_cb+0xd/0x20 [uio_hv_generic]
>>>>>> [   19.646994] Code: 02 00 00 5b 5d e9 53 98 69 e9 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 48 8b 47 10 <48> 8b b8 a0 00 00 00 f0 83 44 24 fc 00 e9 51 6f fa ff 90 90 90 90
>>>>>> [   19.654377] RSP: 0018:ffffb15ac01a4fa8 EFLAGS: 00010046
>>>>>> [   19.656385] RAX: 0000000000000000 RBX: 0000000000000015 RCX: 0000000000000015
>>>>>> [   19.659240] RDX: 0000000000000001 RSI: ffffffffffffffff RDI: ffff8ff69c759400
>>>>>> [   19.662168] RBP: ffff8ff548790200 R08: ffff8ff548790200 R09: 00fca75150b080e9
>>>>>> [   19.665239] R10: 0000000000000000 R11: ffffb15ac01a4ff8 R12: ffff8ff871dc1480
>>>>>> [   19.668193] R13: ffff8ff69c759400 R14: ffff8ff69c7596a0 R15: ffffffffc106e160
>>>>>> [   19.671106] FS:  0000000000000000(0000) GS:ffff8ff871d80000(0000) knlGS:0000000000000000
>>>>>> [   19.674281] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> [   19.676533] CR2: 00000000000000a0 CR3: 0000000100ba6003 CR4: 00000000003706f0
>>>>>> [   19.679385] Call Trace:
>>>>>> [   19.680361]  <IRQ>
>>>>>> [   19.681181]  vmbus_isr+0x1a5/0x210 [hv_vmbus]
>>>>>> [   19.682916]  __sysvec_hyperv_callback+0x32/0x60
>>>>>> [   19.684991]  sysvec_hyperv_callback+0x6c/0x90
>>>>>> [   19.686665]  </IRQ>
>>>>>> [   19.687509]  <TASK>
>>>>>> [   19.688366]  asm_sysvec_hyperv_callback+0x1a/0x20
>>>>>> [   19.690262] RIP: 0010:pv_native_safe_halt+0xf/0x20
>>>>>> [   19.692067] Code: 09 e9 c5 08 01 00 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 66 90 0f 00 2d e5 3b 31 00 fb f4 <c3> cc cc cc cc 66 66 2e 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90
>>>>>> [   19.699119] RSP: 0018:ffffb15ac0103ed8 EFLAGS: 00000246
>>>>>> [   19.701412] RAX: 0000000000000003 RBX: ffff8ff5403b1fc0 RCX: ffff8ff54c64ce30
>>>>>> [   19.704328] RDX: 0000000000000000 RSI: 0000000000000003 RDI: 000000000001f894
>>>>>> [   19.706910] RBP: 0000000000000003 R08: 000000000bb760d9 R09: 00fca75150b080e9
>>>>>> [   19.709762] R10: 0000000000000003 R11: 0000000000000001 R12: 0000000000000000
>>>>>> [   19.712510] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
>>>>>> [   19.715173]  default_idle+0x9/0x20
>>>>>> [   19.716846]  default_idle_call+0x29/0x100
>>>>>> [   19.718623]  do_idle+0x1fe/0x240
>>>>>> [   19.720045]  cpu_startup_entry+0x29/0x30
>>>>>> [   19.721595]  start_secondary+0x11e/0x140
>>>>>> [   19.723080]  common_startup_64+0x13e/0x141
>>>>>> [   19.725222]  </TASK>
>>>>>> [   19.726387] Modules linked in: isofs cdrom uio_hv_generic uio binfmt_misc intel_rapl_msr intel_rapl_common intel_uncore_frequency_common isst_if_mbox_msr isst_if_common rpcrdma skx_edac_common nfit sunrpc libnvdimm crct10dif_pclmul ghash_clmulni_intel sha512_ssse3 sha256_ssse3 rdma_ucm ib_iser sha1_ssse3 rdma_cm aesni_intel iw_cm gf128mul crypto_simd libiscsi cryptd ib_umad ib_ipoib scsi_transport_iscsi ib_cm rapl sg hv_utils hv_balloon evdev pcspkr joydev mpls_router ip_tunnel ramoops configfs pstore_blk efi_pstore pstore_zone nfnetlink vsock_loopback vmw_vsock_virtio_transport_common hv_sock vmw_vsock_vmci_transport vsock vmw_vmci efivarfs ip_tables x_tables autofs4 overlay squashfs dm_verity dm_bufio reed_solomon dm_mod loop ext4 crc16 mbcache jbd2 crc32c_generic mlx5_ib ib_uverbs ib_core mlx5_core mlxfw pci_hyperv pci_hyperv_intf hyperv_drm drm_shmem_helper sd_mod drm_kms_helper hv_storvsc scsi_transport_fc drm scsi_mod hid_generic hid_hyperv hid serio_raw hv_netvsc hyperv_keyboard scsi_common hv_vmbus
>>>>>> [   19.726466]  crc32_pclmul crc32c_intel
>>>>>> [   19.765771] CR2: 00000000000000a0
>>>>>> [   19.767524] ---[ end trace 0000000000000000 ]---
>>>>>> [   19.800433] RIP: 0010:hv_uio_channel_cb+0xd/0x20 [uio_hv_generic]
>>>>>> [   19.803170] Code: 02 00 00 5b 5d e9 53 98 69 e9 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 0f 1f 44 00 00 48 8b 47 10 <48> 8b b8 a0 00 00 00 f0 83 44 24 fc 00 e9 51 6f fa ff 90 90 90 90
>>>>>> [   19.811041] RSP: 0018:ffffb15ac01a4fa8 EFLAGS: 00010046
>>>>>> [   19.813466] RAX: 0000000000000000 RBX: 0000000000000015 RCX: 0000000000000015
>>>>>> [   19.816504] RDX: 0000000000000001 RSI: ffffffffffffffff RDI: ffff8ff69c759400
>>>>>> [   19.819484] RBP: ffff8ff548790200 R08: ffff8ff548790200 R09: 00fca75150b080e9
>>>>>> [   19.822625] R10: 0000000000000000 R11: ffffb15ac01a4ff8 R12: ffff8ff871dc1480
>>>>>> [   19.825569] R13: ffff8ff69c759400 R14: ffff8ff69c7596a0 R15: ffffffffc106e160
>>>>>> [   19.828804] FS:  0000000000000000(0000) GS:ffff8ff871d80000(0000) knlGS:0000000000000000
>>>>>> [   19.832214] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>>>> [   19.834709] CR2: 00000000000000a0 CR3: 0000000100ba6003 CR4: 00000000003706f0
>>>>>> [   19.837976] Kernel panic - not syncing: Fatal exception in interrupt
>>>>>> [   19.841825] Kernel Offset: 0x28a00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
>>>>>> [   19.896620] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---
>>>>>>
>>>>
>>>> <snip>
>>>>
>>>>> The offending commit appers to be the backport of b15b7d2a1b09
>>>>> ("uio_hv_generic: Let userspace take care of interrupt mask") for
>>>>> 6.12.y.
>>>>>
>>>>> Peter confirmed that reverting this commit on top of 6.12.57-1 as
>>>>> packaged in Debian resolves indeed the issue. Interestingly the issue
>>>>> is *not* seen with 6.17.7 based kernel in Debian.
>>>>>
>>>>> #regzbot introduced: 37bd91f22794dc05436130d6983302cb90ecfe7e
>>>>> #regzbot monitor: https://bugs.debian.org/1120602
>>>>>
>>>>> Thank you already!
>>>>>
>>>>> Regards,
>>>>> Salvatore
>>>>
>>>> Hi Peter, Salvatore,
>>>> Thanks for reporting this crash, and sorry for the trouble. Here is my
>>>> analysis.
>>>>
>>>> On 6.17.7, where commit d062463edf17 ("uio_hv_generic: Set event for all
>>>> channels on the device") is present, hv_uio_irqcontrol() supports
>>>> setting of interrupt mask from userspace for sub-channels as well.
>>>>
>>>> This aligns with commit e29587c07537 ("uio_hv_generic: Let userspace
>>>> take care of interrupt mask") which relies on userspace to manage
>>>> interrupt mask, so it safely removes the interrupt mask management logic
>>>> in the driver.
>>>>
>>>> However, in 6.12.57, the first commit is not present, but the second one
>>>> is, so there is no way to disable interrupt mask for sub-channels and
>>>> interrupt_mask stays 0, which means interrupts are not masked. So we may
>>>> be having an interrupt callback being handled for a sub-channel, where
>>>> we do not expect it to come. This may be causing this issue.
>>>>
>>>> This would have led to a crash in hv_uio_channel_cb() for sub-channels:
>>>> struct hv_device *hv_dev = chan->device_obj;
>>>>
>>>>
>>>> I have ported commit d062463edf17 ("uio_hv_generic: Set event for all
>>>> channels on the device") on 6.12.57, and resolved some merge conflicts.
>>>> Could you please help with testing this, if it works for you.
>>>
>>> Applying the patch against the debian 6.12.57 kernel worked, I am no
>>> longer seeing that panic on boot:
>>>
>>> gnos@...ge:~$ uname -a
>>> Linux vEdge 6.12+unreleased-amd64 #1 SMP PREEMPT_DYNAMIC Debian
>>> 6.12.57-1a~test (2025-11-14) x86_64 GNU/Linux
>>> gnos@...ge:~$ uptime
>>>    11:46:33 up 4 min,  1 user,  load average: 3.31, 2.07, 0.89
>>> gnos@...ge:~$ sudo dmidecode -t system
>>> # dmidecode 3.6
>>> Getting SMBIOS data from sysfs.
>>> SMBIOS 3.1.0 present.
>>>
>>> Handle 0x0001, DMI type 1, 27 bytes
>>> System Information
>>>           Manufacturer: Microsoft Corporation
>>>           Product Name: Virtual Machine
>>>           Version: Hyper-V UEFI Release v4.1
>>>           Serial Number: 0000-0002-8036-1108-7588-3134-50
>>>           UUID: 26e86d6e-140c-496a-862c-a3b3bbcd16ad
>>>           Wake-up Type: Power Switch
>>>           SKU Number: None
>>>           Family: Virtual Machine
>>>
>>> Handle 0x0010, DMI type 32, 11 bytes
>>> System Boot Information
>>>           Status: No errors detected
>>>
>>> gnos@...ge:~$
>>>
>>> Thanks a lot for the quick analysis!
>>>
>>> Peter.
>>
>> Hi Peter,
>>
>> Thanks for confirming. I am discussing this with Long Li, to hear his
>> thoughts on this, and have kept the patch ready.
>> Porting the same on 6.6 and older kernels would be a little different since
>> we don't have commit 547fa4ffd799 ("uio_hv_generic: Enable interrupt for low
>> speed VMBus devices") on these kernels and this would lead to merge
>> conflicts, which needs to be handled separately.
>>
>> Meanwhile, if I should be including any tags in the fix patch for debian
>> bug, please let me know.
> 
> Thank you very much for the quick analysis and fix.
> 
> If you can add a Closes: https://bugs.debian.org/1120602 that would
> make our tracking for the fixes easier. But not sure if this is
> allowed for proposing the backport for a stable series, as it did not
> affect the upper releases.
> 
> In any case your work is much appreciated!
> 
> Regards,
> Salvatore

Hi,
I have sent the patches now to the list. Please consider adding your 
tested-by if you find it alright.

Thanks.

Regards,
Naman

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