[PATCH 1/1] drm: check for NULL pointer in drm_gem_object_put

[Christian König]

Am 20.05.20 um 14:49 schrieb Chris Wilson:
> Quoting Christian König (2020-05-20 13:19:42)
>> Am 20.05.20 um 14:14 schrieb Nirmoy Das:
>>> drm_gem_fb_destroy() calls drm_gem_object_put() with NULL obj causing:
>>> [   11.584209] BUG: kernel NULL pointer dereference, address: 0000000000000000
>>> [   11.584213] #PF: supervisor write access in kernel mode
>>> [   11.584215] #PF: error_code(0x0002) - not-present page
>>> [   11.584216] PGD 0 P4D 0
>>> [   11.584220] Oops: 0002 [#1] SMP NOPTI
>>> [   11.584223] CPU: 7 PID: 1571 Comm: gnome-shell Tainted: G            E     5.7.0-rc1-1-default+ #27
>>> [   11.584225] Hardware name: Micro-Star International Co., Ltd. MS-7A31/X370 XPOWER GAMING TITANIUM (MS-7A31), BIOS 1.MR 12/03/2019
>>> [   11.584237] RIP: 0010:drm_gem_fb_destroy+0x28/0x70 [drm_kms_helper]
>>> <snip>
>>> [   11.584256] Call Trace:
>>> [   11.584279]  drm_mode_rmfb+0x189/0x1c0 [drm]
>>> [   11.584299]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
>>> [   11.584314]  drm_ioctl_kernel+0xaa/0xf0 [drm]
>>> [   11.584329]  drm_ioctl+0x1ff/0x3b0 [drm]
>>> [   11.584347]  ? drm_mode_rmfb+0x1c0/0x1c0 [drm]
>>> [   11.584421]  amdgpu_drm_ioctl+0x49/0x80 [amdgpu]
>>> [   11.584427]  ksys_ioctl+0x87/0xc0
>>> [   11.584430]  __x64_sys_ioctl+0x16/0x20
>>> [   11.584434]  do_syscall_64+0x5f/0x240
>>> [   11.584438]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>> [   11.584440] RIP: 0033:0x7f0ef80f7227
>>>
>>> Signed-off-by: Nirmoy Das <nirmoy.das at amd.com>
>> Fixes: .... missing here. Apart from that Reviewed-by: Christian König
>> <christian.koenig at amd.com>.
>>
>> Please resend with the tag added, then I'm going to push this to
>> drm-misc-next asap.
>>
>> Christian.
>>
>>> ---
>>>    include/drm/drm_gem.h | 3 +++
>>>    1 file changed, 3 insertions(+)
>>>
>>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>>> index 52173abdf500..a13510346a9b 100644
>>> --- a/include/drm/drm_gem.h
>>> +++ b/include/drm/drm_gem.h
>>> @@ -372,6 +372,9 @@ static inline void drm_gem_object_get(struct drm_gem_object *obj)
>>>    static inline void
>>>    drm_gem_object_put(struct drm_gem_object *obj)
>>>    {
>>> +     if (!obj)
>>> +             return;
>>> +
> This adds several thousand NULL checks where there were previously none.
> I doubt the compiler eliminates them all.
>
> I'd suggest reverting
> b5d250744ccc ("drm/gem: fold drm_gem_object_put_unlocked and __drm_gem_object_put()")

I didn't looked into this patch in detail, but allowing to call *_put() 
functions with NULL and nothing bad happens is rather common practice.

On the other hand I agree that this might cause a performance problem. 
How many sites do we have which could call drm_gem_object_put() with NULL?

Christian.

> -Chris