[PATCH] drm/bridge: megachips: Fix a null pointer dereference bug

[Ian Ray]

On Tue, Jul 19, 2022 at 09:38:16AM +0100, Martyn Welch wrote:
> On Sat, 2022-07-16 at 16:13 +0800, Zheyu Ma wrote:
> > When removing the module we will get the following warning:
> > 
> > [   31.911505] i2c-core: driver [stdp2690-ge-b850v3-fw] unregistered
> > [   31.912484] general protection fault, probably for non-canonical
> > address 0xdffffc0000000001: 0000 [#1] PREEMPT SMP KASAN PTI
> > [   31.913338] KASAN: null-ptr-deref in range [0x0000000000000008-
> > 0x000000000000000f]
> > [   31.915280] RIP: 0010:drm_bridge_remove+0x97/0x130
> > [   31.921825] Call Trace:
> > [   31.922533]  stdp4028_ge_b850v3_fw_remove+0x34/0x60
> > [megachips_stdpxxxx_ge_b850v3_fw]
> > [   31.923139]  i2c_device_remove+0x181/0x1f0
> > 
> > The two bridges (stdp2690, stdp4028) do not probe at the same time,
> > so
> > the driver does not call ge_b850v3_resgiter() when probing, causing
> > the
> > driver to try to remove the object that has not been initialized.
> > 
> > Fix this by checking whether both the bridges are probed.
> > 
> > Signed-off-by: Zheyu Ma <zheyuma97 at gmail.com>
> 
> Good catch!
> 
> Acked-by: Martyn Welch <martyn.welch at collabora.com>

Tested-by: Ian Ray <ian.ray at ge.com>


> 
> Would be worth adding:
> 
> Fixes: 11632d4aa2b3 ("drm/bridge: megachips: Ensure both bridges are
> probed before registration")
> 
> > ---
> >  drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> > b/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> > index cce98bf2a4e7..c68a4cdf4625 100644
> > --- a/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> > +++ b/drivers/gpu/drm/bridge/megachips-stdpxxxx-ge-b850v3-fw.c
> > @@ -296,7 +296,9 @@ static void ge_b850v3_lvds_remove(void)
> >          * This check is to avoid both the drivers
> >          * removing the bridge in their remove() function
> >          */
> > -       if (!ge_b850v3_lvds_ptr)
> > +       if (!ge_b850v3_lvds_ptr ||
> > +               !ge_b850v3_lvds_ptr->stdp2690_i2c ||
> > +               !ge_b850v3_lvds_ptr->stdp4028_i2c)
> >                 goto out;
> >  
> >         drm_bridge_remove(&ge_b850v3_lvds_ptr->bridge);
>