[PATCH] drm: Fix FD ownership check in drm_master_check_perm()

[Christian König]

Am 07.12.23 um 11:12 schrieb Linus Walleij:
> On Wed, Dec 6, 2023 at 2:52 PM Lingkai Dong <Lingkai.Dong at arm.com> wrote:
>
>> The DRM subsystem keeps a record of the owner of a DRM device file
>> descriptor using thread group ID (TGID) instead of process ID (PID), to
>> ensures all threads within the same userspace process are considered the
>> owner. However, the DRM master ownership check compares the current
>> thread's PID against the record, so the thread is incorrectly considered to
>> be not the FD owner if the PID is not equal to the TGID. This causes DRM
>> ioctls to be denied master privileges, even if the same thread that opened
>> the FD performs an ioctl. Fix this by checking TGID.
>>
>> Fixes: 4230cea89cafb ("drm: Track clients by tgid and not tid")
>> Signed-off-by: Lingkai Dong <lingkai.dong at arm.com>
> Paging the patch author (Tvrko) and committer (Christian).
> Here is the patch if you don't have it in your mailbox:
> https://lore.kernel.org/dri-devel/PA6PR08MB107665920BE9A96658CDA04CE8884A@PA6PR08MB10766.eurprd08.prod.outlook.com/
>
> I'm seeing this as well (on Android).
>
> Tvrko, Christian: can you look at this?

Good catch, looks like we missed this occasion while switching from PID 
to TGID.

> Will you apply it to the AMD tree for fixes if it looks OK
> or does it go elsewhere?

I can push this to drm-misc-fixes as long as nobody objects in the next 
hour or so.

CC: stable? If yes which versions?

Regards,
Christian.

>
> Yours,
> Linus Walleij