acpi_get_devices() crash when acpi_disabled==true (was [PATCH v2] drm/privacy-screen: honor acpi=off in detect_thinkpad_privacy_screen)

[Rafael J. Wysocki]

On Wed, Jan 26, 2022 at 2:47 PM Hans de Goede <hdegoede at redhat.com> wrote:
>
> Hi All,
>
> On 1/23/22 10:10, Tong Zhang wrote:
> > when acpi=off is provided in bootarg, kernel crash with
> >
> > [    1.252739] BUG: kernel NULL pointer dereference, address: 0000000000000018
> > [    1.258308] Call Trace:
> > [    1.258490]  ? acpi_walk_namespace+0x147/0x147
> > [    1.258770]  acpi_get_devices+0xe4/0x137
> > [    1.258921]  ? drm_core_init+0xc0/0xc0 [drm]
> > [    1.259108]  detect_thinkpad_privacy_screen+0x5e/0xa8 [drm]
> > [    1.259337]  drm_privacy_screen_lookup_init+0xe/0xe85 [drm]
> >
> > The reason is that acpi_walk_namespace expects acpi related stuff
> > initialized but in fact it wouldn't when acpi is set to off. In this case
> > we should honor acpi=off in detect_thinkpad_privacy_screen().
> >
> > Signed-off-by: Tong Zhang <ztong0001 at gmail.com>
>
> Thank you for catching this and thank you for your patch. I was about to merge
> this, but then I realized that this might not be the best way to fix this.
>
> A quick grep shows 10 acpi_get_devices() calls outside of drivers/acpi,
> and at a first glance about half of those are missing an acpi_disabled
> check. IMHO it would be better to simply add an acpi_disabled check to
> acpi_get_devices() itself.
>
> Rafael, do you agree ?

Yes, I do.

> Note the just added chrome privacy-screen check uses
> acpi_dev_present(), this is also used in about 10 places outside
> of drivers/acpi and AFAIK none of those do an acpi_disabled check.
>
> acpi_dev_present() uses bus_find_device(&acpi_bus_type, ...)
> but the acpi_bus_type does not get registered when acpi_disabled
> is set. In the end this is fine though since bus_find_device
> checks for the bus not being registered and then just returns
> NULL.

Right.

> > ---
> > v2: fix typo in previous commit -- my keyboard is eating letters
> >
> >  drivers/gpu/drm/drm_privacy_screen_x86.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/drm_privacy_screen_x86.c b/drivers/gpu/drm/drm_privacy_screen_x86.c
> > index a2cafb294ca6..e7aa74ad0b24 100644
> > --- a/drivers/gpu/drm/drm_privacy_screen_x86.c
> > +++ b/drivers/gpu/drm/drm_privacy_screen_x86.c
> > @@ -33,6 +33,9 @@ static bool __init detect_thinkpad_privacy_screen(void)
> >       unsigned long long output;
> >       acpi_status status;
> >
> > +     if (acpi_disabled)
> > +             return false;
> > +
> >       /* Get embedded-controller handle */
> >       status = acpi_get_devices("PNP0C09", acpi_set_handle, NULL, &ec_handle);
> >       if (ACPI_FAILURE(status) || !ec_handle)
>