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

[Michael Kelley]

From: Easwar Hariharan <easwar.hariharan@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx>
> > Closes: https://lore.kernel.org/linux-
> hyperv/20260721154943.A09BD1F00A3D@xxxxxxxxxxxxxxx/
> > Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if Linux is root")
> > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > ---
> >  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@xxxxxxxxxxxxxxxxxxx>