On 8/4/2026 15:17, Michael Kelley wrote: > From: Easwar Hariharan <[email protected]> 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 <[email protected]> >>> Closes: https://lore.kernel.org/linux- >> hyperv/[email protected]/ >>> Fixes: 7e279d78664aa ("Drivers: hv: vmbus: skip VMBus initialization if >>> Linux is root") >>> Signed-off-by: Michael Kelley <[email protected]> >>> --- >>> 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)
