[PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Michael Kelley]

The VMBus module should not be loaded when Linux is running directly
in the root partition and root is not nested in another VM. Current
code checks this condition and skips VMBus module initialization, which
works. But it returns 0 as the result, so Linux thinks the module has
successfully loaded. Later, if the module were to be unloaded, the
VMBus module unload code tries to clean up things that were never
initialized, resulting in memory faults and a panic.

Fix this by having VMBus module initialization return -ENODEV for this
case. The module is then not loaded, and the unload path can never run.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/linux-hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
 drivers/hv/vmbus_drv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index e19ec73b0187..849d7e1a7320 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
 		return -ENODEV;
 
 	if (hv_root_partition() && !hv_nested)
-		return 0;
+		return -ENODEV;
 
 	/*
 	 * Get ACPI resources first.
-- 
2.25.1

Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Returning -ENODEV in the root partition prevents the hv_vmbus module from loading, which breaks the loading of the mshv_root module when both are configured as loadable modules.
--

--- Patch [1]: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition ---
commit 49405a084beb024e2ba8a360ff4836611b21617d
Author: Michael Kelley <mhkelley58@gmail.com>

Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

This commit changes the VMBus module initialization to return -ENODEV instead
of 0 when running directly in the root partition and not nested. This is
intended to prevent the module from successfully loading and later causing
memory faults or a panic during unload when resources were never initialized.

Closes: https://lore.kernel.org/linux-hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/

> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index 6824bd7cb3c4f..7d5021ceea84f 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -2982,7 +2982,7 @@ static int __init hv_acpi_init(void)
>  		return -ENODEV;
>  
>  	if (hv_root_partition() && !hv_nested)
> -		return 0;
> +		return -ENODEV;

[Severity: High]
Does returning -ENODEV here break dependent modules when both are built as
loadable modules?

If the VMBus initialization returns an error on a non-nested root partition,
the hv_vmbus module will fail to load and its symbols (like hv_vmbus_exists())
will not be exported.

This would prevent the mshv_root module from loading on the root partition
since it depends on those exported symbols, completely breaking mshv_root
functionality in its only intended environment.

>  
>  	/*
>  	 * Get ACPI resources first.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260804190517.101981-1-mhklinux@outlook.com?part=1

Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Easwar Hariharan]

On 8/4/2026 12:05, Michael Kelley wrote:
> The VMBus module should not be loaded when Linux is running directly
> in the root partition and root is not nested in another VM. Current
> code checks this condition and skips VMBus module initialization, which
> works. But it returns 0 as the result, so Linux thinks the module has
> successfully loaded. Later, if the module were to be unloaded, the
> VMBus module unload code tries to clean up things that were never
> initialized, resulting in memory faults and a panic.
> 
> Fix this by having VMBus module initialization return -ENODEV for this
> case. The module is then not loaded, and the unload path can never run.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/linux-hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> ---
>  drivers/hv/vmbus_drv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index e19ec73b0187..849d7e1a7320 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
>  		return -ENODEV;
>  
>  	if (hv_root_partition() && !hv_nested)
> -		return 0;
> +		return -ENODEV;
>  
>  	/*
>  	 * Get ACPI resources first.

This seems straightforward:


Reviewed-by: Easwar Hariharan <easwar.hariharan@linux.microsoft.com>

RE: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Michael Kelley]

From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 12:40 PM
> 
> On 8/4/2026 12:05, Michael Kelley wrote:
> > The VMBus module should not be loaded when Linux is running directly
> > in the root partition and root is not nested in another VM. Current
> > code checks this condition and skips VMBus module initialization, which
> > works. But it returns 0 as the result, so Linux thinks the module has
> > successfully loaded. Later, if the module were to be unloaded, the
> > VMBus module unload code tries to clean up things that were never
> > initialized, resulting in memory faults and a panic.
> >
> > Fix this by having VMBus module initialization return -ENODEV for this
> > case. The module is then not loaded, and the unload path can never run.
> >
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Closes: https://lore.kernel.org/linux-
> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
> > Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
> > Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> > ---
> >  drivers/hv/vmbus_drv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> > index e19ec73b0187..849d7e1a7320 100644
> > --- a/drivers/hv/vmbus_drv.c
> > +++ b/drivers/hv/vmbus_drv.c
> > @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
> >  		return -ENODEV;
> >
> >  	if (hv_root_partition() && !hv_nested)
> > -		return 0;
> > +		return -ENODEV;
> >
> >  	/*
> >  	 * Get ACPI resources first.
> 
> This seems straightforward:

Alas, it's not so straightforward, as Sashiko pointed out.  I knew that
the mshv module has a dependency on the vmbus module, but had
forgotten. There's a reason for the dependency as described in the
commit message for 840b740a35bf.

There's another easy way to fix the VMBus module unload problem.
I'll send a v2. :-)

Michael

> 
> 
> Reviewed-by: Easwar Hariharan <easwar.hariharan@linux.microsoft.com>

Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Easwar Hariharan]

On 8/4/2026 15:17, Michael Kelley wrote:
> From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 12:40 PM
>>
>> On 8/4/2026 12:05, Michael Kelley wrote:
>>> The VMBus module should not be loaded when Linux is running directly
>>> in the root partition and root is not nested in another VM. Current
>>> code checks this condition and skips VMBus module initialization, which
>>> works. But it returns 0 as the result, so Linux thinks the module has
>>> successfully loaded. Later, if the module were to be unloaded, the
>>> VMBus module unload code tries to clean up things that were never
>>> initialized, resulting in memory faults and a panic.
>>>
>>> Fix this by having VMBus module initialization return -ENODEV for this
>>> case. The module is then not loaded, and the unload path can never run.
>>>
>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
>>> Closes: https://lore.kernel.org/linux-
>> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
>>> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
>>> ---
>>>  drivers/hv/vmbus_drv.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
>>> index e19ec73b0187..849d7e1a7320 100644
>>> --- a/drivers/hv/vmbus_drv.c
>>> +++ b/drivers/hv/vmbus_drv.c
>>> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
>>>  		return -ENODEV;
>>>
>>>  	if (hv_root_partition() && !hv_nested)
>>> -		return 0;
>>> +		return -ENODEV;
>>>
>>>  	/*
>>>  	 * Get ACPI resources first.
>>
>> This seems straightforward:
> 
> Alas, it's not so straightforward, as Sashiko pointed out.  I knew that
> the mshv module has a dependency on the vmbus module, but had
> forgotten. There's a reason for the dependency as described in the
> commit message for 840b740a35bf.
> 
> There's another easy way to fix the VMBus module unload problem.
> I'll send a v2. :-)
> 
> Michael
> 
I may be missing something, but mshv_root is used in 3 cases:

1) Baremetal root partition, which requires no VMBus

2) L1VH aka Direct Virtualization, which is conditioned on hv_l1vh_partition(), which is not the check here in the vmbus driver

3) For the OpenHCL paravisor, where I honestly don't know what the dependency chain looks like, but based on a quick glance
at https://github.com/microsoft/OHCL-Linux-Kernel/ and a cursory grep, doesn't seem to rely on hv_root_partition() but does rely
on VMbus.

I feel like Sashiko's review falls into item 1, but then again, there may just be a mismatch
between reality and my mental model, or my mental model may becorrect, but the code doesn't match it.

Thanks,
Easwar (he/him)

Re: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Easwar Hariharan]

On 8/4/2026 15:47, Easwar Hariharan wrote:
> On 8/4/2026 15:17, Michael Kelley wrote:
>> From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 12:40 PM
>>>
>>> On 8/4/2026 12:05, Michael Kelley wrote:
>>>> The VMBus module should not be loaded when Linux is running directly
>>>> in the root partition and root is not nested in another VM. Current
>>>> code checks this condition and skips VMBus module initialization, which
>>>> works. But it returns 0 as the result, so Linux thinks the module has
>>>> successfully loaded. Later, if the module were to be unloaded, the
>>>> VMBus module unload code tries to clean up things that were never
>>>> initialized, resulting in memory faults and a panic.
>>>>
>>>> Fix this by having VMBus module initialization return -ENODEV for this
>>>> case. The module is then not loaded, and the unload path can never run.
>>>>
>>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
>>>> Closes: https://lore.kernel.org/linux-
>>> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
>>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
>>>> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
>>>> ---
>>>>  drivers/hv/vmbus_drv.c | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
>>>> index e19ec73b0187..849d7e1a7320 100644
>>>> --- a/drivers/hv/vmbus_drv.c
>>>> +++ b/drivers/hv/vmbus_drv.c
>>>> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
>>>>  		return -ENODEV;
>>>>
>>>>  	if (hv_root_partition() && !hv_nested)
>>>> -		return 0;
>>>> +		return -ENODEV;
>>>>
>>>>  	/*
>>>>  	 * Get ACPI resources first.
>>>
>>> This seems straightforward:
>>
>> Alas, it's not so straightforward, as Sashiko pointed out.  I knew that
>> the mshv module has a dependency on the vmbus module, but had
>> forgotten. There's a reason for the dependency as described in the
>> commit message for 840b740a35bf.
>>
>> There's another easy way to fix the VMBus module unload problem.
>> I'll send a v2. :-)
>>
>> Michael
>>
> I may be missing something, but mshv_root is used in 3 cases:
> 
> 1) Baremetal root partition, which requires no VMBus
> 
> 2) L1VH aka Direct Virtualization, which is conditioned on hv_l1vh_partition(), which is not the check here in the vmbus driver
> 
> 3) For the OpenHCL paravisor, where I honestly don't know what the dependency chain looks like, but based on a quick glance
> at https://github.com/microsoft/OHCL-Linux-Kernel/ and a cursory grep, doesn't seem to rely on hv_root_partition() but does rely
> on VMbus.
> 
> I feel like Sashiko's review falls into item 1, but then again, there may just be a mismatch
> between reality and my mental model, or my mental model may becorrect, but the code doesn't match it.
> 
> Thanks,
> Easwar (he/him)


Well, number 4 is as a parent on a nested hypervisor, but !hv_nested takes care of that.

- Easwar

RE: [PATCH 1/1] Drivers: hv: vmbus: Fail VMBus module init for non-nested root partition

[Michael Kelley]

From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 3:49 PM
> 
> On 8/4/2026 15:47, Easwar Hariharan wrote:
> > On 8/4/2026 15:17, Michael Kelley wrote:
> >> From: Easwar Hariharan <easwar.hariharan@linux.microsoft.com> Sent: Tuesday, August 4, 2026 12:40 PM
> >>>
> >>> On 8/4/2026 12:05, Michael Kelley wrote:
> >>>> The VMBus module should not be loaded when Linux is running directly
> >>>> in the root partition and root is not nested in another VM. Current
> >>>> code checks this condition and skips VMBus module initialization, which
> >>>> works. But it returns 0 as the result, so Linux thinks the module has
> >>>> successfully loaded. Later, if the module were to be unloaded, the
> >>>> VMBus module unload code tries to clean up things that were never
> >>>> initialized, resulting in memory faults and a panic.
> >>>>
> >>>> Fix this by having VMBus module initialization return -ENODEV for this
> >>>> case. The module is then not loaded, and the unload path can never run.
> >>>>
> >>>> Reported-by: Sashiko <sashiko-bot@kernel.org>
> >>>> Closes: https://lore.kernel.org/linux-
> >>> hyperv/20260721154943.A09BD1F00A3D@smtp.kernel.org/
> >>>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
> >>>> Signed-off-by: Michael Kelley <mhklinux@outlook.com>
> >>>> ---
> >>>>  drivers/hv/vmbus_drv.c | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> >>>> index e19ec73b0187..849d7e1a7320 100644
> >>>> --- a/drivers/hv/vmbus_drv.c
> >>>> +++ b/drivers/hv/vmbus_drv.c
> >>>> @@ -2976,7 +2976,7 @@ static int __init hv_acpi_init(void)
> >>>>  		return -ENODEV;
> >>>>
> >>>>  	if (hv_root_partition() && !hv_nested)
> >>>> -		return 0;
> >>>> +		return -ENODEV;
> >>>>
> >>>>  	/*
> >>>>  	 * Get ACPI resources first.
> >>>
> >>> This seems straightforward:
> >>
> >> Alas, it's not so straightforward, as Sashiko pointed out.  I knew that
> >> the mshv module has a dependency on the vmbus module, but had
> >> forgotten. There's a reason for the dependency as described in the
> >> commit message for 840b740a35bf.
> >>
> >> There's another easy way to fix the VMBus module unload problem.
> >> I'll send a v2. :-)
> >>
> >> Michael
> >>
> > I may be missing something, but mshv_root is used in 3 cases:
> >
> > 1) Baremetal root partition, which requires no VMBus
> >
> > 2) L1VH aka Direct Virtualization, which is conditioned on hv_l1vh_partition(), which is
> not the check here in the vmbus driver
> >
> > 3) For the OpenHCL paravisor, where I honestly don't know what the dependency
> chain looks like, but based on a quick glance
> > at https://github.com/microsoft/OHCL-Linux-Kernel/ and a cursory grep, doesn't
> seem to rely on hv_root_partition() but does rely
> > on VMbus.
> >
> > I feel like Sashiko's review falls into item 1, but then again, there may just be a
> mismatch
> > between reality and my mental model, or my mental model may becorrect, but the
> code doesn't match it.
> >
> > Thanks,
> > Easwar (he/him)
> 
> 
> Well, number 4 is as a parent on a nested hypervisor, but !hv_nested takes care of that.
> 

For your #1, indeed the root partition does not require actual VMBus
functionality. But it *does* require that the VMBus module be loaded
if CONFIG_HYPERV_VMBUS=y. In that case, the only function the root
requires for #1 is the function hv_vmbus_exists(), and it requires that
to distinguish between your #1 and your #4 when it sets up the SynIC.

If you want to build an image that only works for #1, then build with
CONFIG_HYPERV_VMBUS=n and there's no issue. But if you want an
image that works for #1 or #4, build with CONFIG_HYPERV_VMBUS=y
or =m, and the VMBus module must loaded so that hv_vmbus_exists()
gives the right answer (false for #1, true for #4). Furthermore, the
mshv_root dependency on the VMBus module ensures that the
VMBus module is always loaded before the mshv_root module so
that hv_vmbus_exists() will give an up-to-date answer when
mshv_root asks.

The way the SynIC is managed should be refactored to better
handle #4, as Jork Loeser has proposed. Hopefully such a refactoring
would make the mshv_root->vmbus dependency go away. But until
such a refactoring is done, the dependency is what we have.

I agree that your #2 and #3 are not relevant here.

Michael