[PATCH] drm/ssd130x: Fix an oops when attempting to update a disabled plane

[Geert Uytterhoeven]

Hi Thomas,

On Mon, Jul 17, 2023 at 10:48 AM Thomas Zimmermann <tzimmermann at suse.de> wrote:
> Am 13.07.23 um 18:32 schrieb Javier Martinez Canillas:
> > Geert reports that the following NULL pointer dereference happens for him
> > after commit 49d7d581ceaf ("drm/ssd130x: Don't allocate buffers on each
> > plane update"):
> >
> >      [drm] Initialized ssd130x 1.0.0 20220131 for 0-003c on minor 0
> >      ssd130x-i2c 0-003c: [drm] surface width(128), height(32), bpp(1)
> >      and format(R1   little-endian (0x20203152))
> >      Unable to handle kernel NULL pointer dereference at virtual address 00000000
> >      Oops [#1]
> >      CPU: 0 PID: 1 Comm: swapper Not tainted
> >      6.5.0-rc1-orangecrab-02219-g0a529a1e4bf4 #565
> >      epc : ssd130x_update_rect.isra.0+0x13c/0x340
> >       ra : ssd130x_update_rect.isra.0+0x2bc/0x340
> >      ...
> >      status: 00000120 badaddr: 00000000 cause: 0000000f
> >      [<c0303d90>] ssd130x_update_rect.isra.0+0x13c/0x340
> >      [<c0304200>] ssd130x_primary_plane_helper_atomic_update+0x26c/0x284
> >      [<c02f8d54>] drm_atomic_helper_commit_planes+0xfc/0x27c
> >      [<c02f9314>] drm_atomic_helper_commit_tail+0x5c/0xb4
> >      [<c02f94fc>] commit_tail+0x190/0x1b8
> >      [<c02f99fc>] drm_atomic_helper_commit+0x194/0x1c0
> >      [<c02c5d00>] drm_atomic_commit+0xa4/0xe4
> >      [<c02cce40>] drm_client_modeset_commit_atomic+0x244/0x278
> >      [<c02ccef0>] drm_client_modeset_commit_locked+0x7c/0x1bc
> >      [<c02cd064>] drm_client_modeset_commit+0x34/0x64
> >      [<c0301a78>] __drm_fb_helper_restore_fbdev_mode_unlocked+0xc4/0xe8
> >      [<c0303424>] drm_fb_helper_set_par+0x38/0x58
> >      [<c027c410>] fbcon_init+0x294/0x534
> >      ...
> >
> > The problem is that fbcon calls fbcon_init() which triggers a DRM modeset
> > and this leads to drm_atomic_helper_commit_planes() attempting to commit
> > the atomic state for all planes, even the ones whose CRTC is not enabled.
> >
> > Since the primary plane buffer is allocated in the encoder .atomic_enable
> > callback, this happens after that initial modeset commit and leads to the
> > mentioned NULL pointer dereference.
> >
> > Fix this by not using the default drm_atomic_helper_commit_tail() helper,
> > but instead the drm_atomic_helper_commit_tail_rpm() function that doesn't
> > attempt to commit the atomic state for planes related to inactive CRTCs.
> >
> > Reported-by: Geert Uytterhoeven <geert at linux-m68k.org>
> > Signed-off-by: Javier Martinez Canillas <javierm at redhat.com>

> > --- a/drivers/gpu/drm/solomon/ssd130x.c
> > +++ b/drivers/gpu/drm/solomon/ssd130x.c
> > @@ -795,6 +795,10 @@ static const struct drm_mode_config_funcs ssd130x_mode_config_funcs = {
> >       .atomic_commit = drm_atomic_helper_commit,
> >   };
> >
> > +static const struct drm_mode_config_helper_funcs ssd130x_mode_config_helpers = {
> > +     .atomic_commit_tail = drm_atomic_helper_commit_tail_rpm,
> > +};
> > +
>
> After some discussion on IRC, I'd suggest to allocate the buffer
> somewhere within probe. So it will always be there when the plane code runs.
>
> A full fix would be to allocate the buffer memory as part of the plane
> state and/or the plane's atomic_check. That's a bit more complicated if
> you want to shared the buffer memory across plane updates.

Note that actually two buffers are involved: data_array (monochrome,
needed for each update), and buffer (R8, only needed when converting
from XR24 to R1).

For the former, I agree, as it's always needed.
For the latter, I'm afraid it would set a bad example: while allocating
a possibly-unused buffer doesn't hurt for small displays, it would
mean wasting 1 MiB in e.g. the repaper driver (once it has gained
support for R1 ;^).

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds