Re: [PATCH] ACPI: scan: Fix a Hyper-V Linux VM panic caused by buffer overflow

[Rafael J. Wysocki]

On Tue, Jan 5, 2021 at 11:02 PM Dexuan Cui <decui@xxxxxxxxxxxxx> wrote:
>
> > From: Michael Kelley <mikelley@xxxxxxxxxxxxx>
> > Sent: Tuesday, December 22, 2020 5:56 AM
> > From: Dexuan Cui
> > Sent: Thursday, December 17, 2020
> > 8:08 PM
> > >
> > > Linux VM on Hyper-V crashes with the latest mainline:
> > > ...
> > > --- a/drivers/acpi/scan.c
> > > +++ b/drivers/acpi/scan.c
> > > @@ -674,7 +674,8 @@ int acpi_device_add(struct acpi_device *device,
> > >     }
> > >     if (!found) {
> > >             acpi_device_bus_id = new_bus_id;
> > > -           strcpy(acpi_device_bus_id->bus_id, acpi_device_hid(device));
> > > +           strlcpy(acpi_device_bus_id->bus_id, acpi_device_hid(device),
> > > +                   sizeof(acpi_device_bus_id->bus_id));
> > >             acpi_device_bus_id->instance_no = 0;
> > >             list_add_tail(&acpi_device_bus_id->node, &acpi_bus_id_list);
> > >     }
> >
> > Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
>
> Hi, ACPI maintainers,
> Would you please take a look at the small fix? Currently the mainline Linux
> kernel, running in a VM on Hyper-V, has been broken for almost 3 weeks,
> i.e. the VM always panics when it boots.

The root cause is a VM issue AFAICS, though.

> The patch has already had Michael's Reviewed-by.
>
> BTW, the patch should have a stable tag:
> Cc: <stable@xxxxxxxxxxxxxxx>
>
> Or, do you want the patch to go through the Hyper-V tree?
> https://git.kernel.org/pub/scm/linux/kernel/git/hyperv/linux.git/log/?h=hyperv-fixes
>
> The small patch is unlikely to cause a merge conflict, and it only affects
> Linux VMs on Hyper-V so far.

It doesn't look like the right fix to me, though.

The problem appears to be that the string coming from _HID is too long
(which is a spec violation).  The patch truncates it to match the
length of the target buffer, but that is not particularly useful.

It would be better to use something like kstrdup_const() to initialize
acpi_device_bus_id->bus_id IMV.