Re: [REGRESSION 6.12.y] hyper-v: BUG: kernel NULL pointer dereference, address: 00000000000000a0: RIP: 0010:hv_uio_channel_cb+0xd/0x20 [uio_hv_generic]

[Naman Jain <namjain@linux.microsoft.com>]

On 11/14/2025 5:19 PM, Peter Morrow wrote:
> Hi Naman,
> 
> On Fri, 14 Nov 2025 at 06:03, Naman Jain <namjain@linux.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@vEdge:~$ 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@vEdge:~$ uptime
>   11:46:33 up 4 min,  1 user,  load average: 3.31, 2.07, 0.89
> gnos@vEdge:~$ 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@vEdge:~$
> 
> 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.

Regards,
Naman