Re: [PATCH] drm/fb-helper: fix leaks in error path of drm_fb_helper_fbdev_setup

[Peter Wu <peter@lekensteyn.nl>]

On Sun, Dec 23, 2018 at 02:55:52PM +0100, Noralf Trønnes wrote:
> 
> 
> Den 23.12.2018 01.55, skrev Peter Wu:
> > After drm_fb_helper_fbdev_setup calls drm_fb_helper_init,
> > "dev->fb_helper" will be initialized (and thus drm_fb_helper_fini will
> > have some effect). After that, drm_fb_helper_initial_config is called
> > which may call the "fb_probe" driver callback.
> > 
> > This driver callback may call drm_fb_helper_defio_init (as is done by
> > drm_fb_helper_generic_probe) or set a framebuffer (as is done by bochs)
> > as documented. These are normally cleaned up on exit by
> > drm_fb_helper_fbdev_teardown which also calls drm_fb_helper_fini.
> > 
> > If an error occurs after "fb_probe", but before setup is complete, then
> > calling just drm_fb_helper_fini will leak resources. This was triggered
> > by df2052cc922 ("bochs: convert to drm_fb_helper_fbdev_setup/teardown"):
> > 
> >      [   50.008030] bochsdrmfb: enable CONFIG_FB_LITTLE_ENDIAN to support this framebuffer
> >      [   50.009436] bochs-drm 0000:00:02.0: [drm:drm_fb_helper_fbdev_setup] *ERROR* fbdev: Failed to set configuration (ret=-38)
> >      [   50.011456] [drm] Initialized bochs-drm 1.0.0 20130925 for 0000:00:02.0 on minor 2
> >      [   50.013604] WARNING: CPU: 1 PID: 1 at drivers/gpu/drm/drm_mode_config.c:477 drm_mode_config_cleanup+0x280/0x2a0
> >      [   50.016175] CPU: 1 PID: 1 Comm: swapper/0 Tainted: G                T 4.20.0-rc7 #1
> >      [   50.017732] EIP: drm_mode_config_cleanup+0x280/0x2a0
> >      ...
> >      [   50.023155] Call Trace:
> >      [   50.023155]  ? bochs_kms_fini+0x1e/0x30
> >      [   50.023155]  ? bochs_unload+0x18/0x40
> > 
> > This can be reproduced with QEMU and CONFIG_FB_LITTLE_ENDIAN=n.
> > 
> > Link: https://lkml.kernel.org/r/20181221083226.GI23332@shao2-debian
> > Link: https://lkml.kernel.org/r/20181223004315.GA11455@al
> > Fixes: 8741216396b2 ("drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()")
> > Reported-by: kernel test robot <rong.a.chen@intel.com>
> > Cc: Noralf Trønnes <noralf@tronnes.org>
> > Signed-off-by: Peter Wu <peter@lekensteyn.nl>
> > ---
> >   drivers/gpu/drm/drm_fb_helper.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> > index 9d64f874f965..432e0f3b9267 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -2860,7 +2860,7 @@ int drm_fb_helper_fbdev_setup(struct drm_device *dev,
> >   	return 0;
> >   err_drm_fb_helper_fini:
> > -	drm_fb_helper_fini(fb_helper);
> > +	drm_fb_helper_fbdev_teardown(dev);
> 
> This change will break the error path for drm_fbdev_generic_setup()
> because drm_fb_helper_generic_probe() cleans up on error but doesn't
> clear drm_fb_helper->fb resulting in a double drm_framebuffer_remove().

This should probably considered a bug of drm_fb_helper_generic_probe.
Ownership of fb_helper should remain with the caller. The caller can
detect an error and act accordingly.

> My assumption has been that the drm_fb_helper_funcs->fb_probe callback
> cleans up its resources on error. Clearly this is not the case for bochs, so
> my take on this is that bochsfb_create() needs to clean up on error.

That assumption still holds for bochs. The problem is this sequence:
- drm_fb_helper_fbdev_setup is called.
- fb_probe succeeds (this is crucial).
- register_framebuffer fails.
- error path of setup is triggered.

As fb_helper is fully setup by drivers, the drm_fb_helper core should
fully deallocate it again on the error path or else a leak occurs.

> Gerd has a patchset that switches bochs over to the generic fbdev
> emulation, but ofc that doesn't help with 4.20:
> https://patchwork.freedesktop.org/series/54269/

And that does not help with other users of the drm_fb_helper who use
functions like drm_fb_helper_defio_init. They will likely run in the
same problem.

I don't have a way to test tinydrm or other drivers, but if you force
register_framebuffer to fail, you should be able to reproduce the
problem with drm_fb_helper_generic_probe.
-- 
Kind regards,
Peter Wu
https://lekensteyn.nl