Message-ID: <7a38c04d-4e54-4f1a-96fd-43f0f11ab97b@linux.microsoft.com>
Date: Fri, 14 Nov 2025 11:33:45 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: Salvatore Bonaccorso <carnil@...ian.org>,
 Peter Morrow <pdmorrow@...il.com>, Long Li <longli@...rosoft.com>
Cc: 1120602@...s.debian.org, 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>, Long Li <longli@...rosoft.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: [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 11/13/2025 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.

Hi Long,
If this works, do you see any concerns if I back-port your patch on 
older kernels (6.12 and prior)?

Regards,
Naman

--------------
Patch:

 From 2f14d48d2bde3f86b153b9f756a9cd688cda3453 Mon Sep 17 00:00:00 2001
From: Long Li <longli@...rosoft.com>
Date: Mon, 10 Mar 2025 15:12:01 -0700
Subject: [PATCH] uio_hv_generic: Set event for all channels on the device

Hyper-V may offer a non latency sensitive device with subchannels without
monitor bit enabled. The decision is entirely on the Hyper-V host not
configurable within guest.

When a device has subchannels, also signal events for the subchannel
if its monitor bit is disabled.

This patch also removes the memory barrier when monitor bit is enabled
as it is not necessary. The memory barrier is only needed between
setting up interrupt mask and calling vmbus_set_event() when monitor
bit is disabled.

Signed-off-by: Long Li <longli@...rosoft.com>
Reviewed-by: Michael Kelley <mhklinux@...look.com>
Reviewed-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>
---
  drivers/uio/uio_hv_generic.c | 32 ++++++++++++++++++++++++++------
  1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/drivers/uio/uio_hv_generic.c b/drivers/uio/uio_hv_generic.c
index 0b414d1168dd..9f3b124a5e09 100644
--- a/drivers/uio/uio_hv_generic.c
+++ b/drivers/uio/uio_hv_generic.c
@@ -65,6 +65,16 @@ struct hv_uio_private_data {
         char    send_name[32];
  };

+static void set_event(struct vmbus_channel *channel, s32 irq_state)
+{
+       channel->inbound.ring_buffer->interrupt_mask = !irq_state;
+       if (!channel->offermsg.monitor_allocated && irq_state) {
+               /* MB is needed for host to see the interrupt mask first */
+               virt_mb();
+               vmbus_set_event(channel);
+       }
+}
+
  /*
   * This is the irqcontrol callback to be registered to uio_info.
   * It can be used to disable/enable interrupt from user space processes.
@@ -79,12 +89,15 @@ hv_uio_irqcontrol(struct uio_info *info, s32 irq_state)
  {
         struct hv_uio_private_data *pdata = info->priv;
         struct hv_device *dev = pdata->device;
+       struct vmbus_channel *primary, *sc;

-       dev->channel->inbound.ring_buffer->interrupt_mask = !irq_state;
-       virt_mb();
+       primary = dev->channel;
+       set_event(primary, irq_state);

-       if (!dev->channel->offermsg.monitor_allocated && irq_state)
-               vmbus_setevent(dev->channel);
+       mutex_lock(&vmbus_connection.channel_mutex);
+       list_for_each_entry(sc, &primary->sc_list, sc_list)
+               set_event(sc, irq_state);
+       mutex_unlock(&vmbus_connection.channel_mutex);

         return 0;
  }
@@ -95,11 +108,18 @@ hv_uio_irqcontrol(struct uio_info *info, s32 irq_state)
  static void hv_uio_channel_cb(void *context)
  {
         struct vmbus_channel *chan = context;
-       struct hv_device *hv_dev = chan->device_obj;
-       struct hv_uio_private_data *pdata = hv_get_drvdata(hv_dev);
+       struct hv_device *hv_dev;
+       struct hv_uio_private_data *pdata;

         virt_mb();

+       /*
+       * The callback may come from a subchannel, in which case look
+       * for the hv device in the primary channel
+       */
+       hv_dev = chan->primary_channel ?
+       chan->primary_channel->device_obj : chan->device_obj;
+       pdata = hv_get_drvdata(hv_dev);
         uio_event_notify(&pdata->info);
  }

--
2.43.0

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