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

From: Noralf TrÃnnes
Date: Mon Dec 24 2018 - 09:59:04 EST

Den 24.12.2018 00.10, skrev Peter Wu:
On Sun, Dec 23, 2018 at 02:55:52PM +0100, Noralf TrÃnnes 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.

Fixes: 8741216396b2 ("drm/fb-helper: Add drm_fb_helper_fbdev_setup/teardown()")
Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
Cc: Noralf TrÃnnes <noralf@xxxxxxxxxxx>
Signed-off-by: Peter Wu <peter@xxxxxxxxxxxxx>
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;
- 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:

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.

Now I understand. I have looked at the drivers that use drm_fb_helper
and no one seem to handle the case where register_framebuffer() is

Here's what drivers do when drm_fb_helper_initial_config() fails:

Doesn't check:

Calls drm_fb_helper_fini():
bochs - Uses drm_fb_helper_fbdev_setup()
qxl - Uses drm_fb_helper_fbdev_setup()
vboxvideo - Uses drm_fb_helper_fbdev_setup()

Might clean up, not sure:

Looks suspicious:

I looked at bochs before it switched to drm_fb_helper_fbdev_setup() and
it also just called drm_fb_helper_fini().

It looks like you've uncovered something no one has though about (or
not implemented at least).

It's not just the framebuffer that's not destroyed, the buffer object
is also leaked. drm_mode_config_cleanup() yells about the framebuffer
(and frees it), but says nothing about the buffer object. It might be
that it can't even be made to detect that since some drivers do special
stuff for the fbdev buffer.

I'll pick up on this and do some testing after the Christmas holidays.