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

["Noralf Trønnes" <noralf@tronnes.org>]

Den 05.01.2019 19.25, skrev Noralf Trønnes:
> 
> 
> Den 24.12.2018 16.03, skrev Peter Wu:
>> On Mon, Dec 24, 2018 at 03:52:55PM +0100, Noralf Trønnes wrote:
>>>
>>>
>>> Den 24.12.2018 00.10, skrev Peter Wu:
>>>> 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.
>>>>
>>>
>>> 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
>>> failing.
>>>
>>> Here's what drivers do when drm_fb_helper_initial_config() fails:
>>>
>>> Doesn't check:
>>> amdgpu
>>> virtio
>>>
>>> Calls drm_fb_helper_fini():
>>> armada
>>> ast
>>> exynos
>>> gma500
>>> hisilicon
>>> mgag200
>>> msm
>>> nouveau
>>> omap
>>> radeon
>>> rockchip
>>> tegra
>>> udl
>>> 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:
>>> cirrus
>>>
>>> Looks suspicious:
>>> i915
>>>
>>> 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.
>>
>> Thanks, the warning is bad for CI (which uses QEMU), but otherwise it
>> should not have any effect on regular users so it can wait.
>>
> 
> This patch is good as long as it's applied along side the fix[1] to the
> generic emulation:
> 
> Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
> 
> I can apply them both when I get an ack/rb on the other patch.
> 

Applied to drm-misc-next.

Noralf.
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