[BUG] drm/amd/display: possible null-pointer dereference or redundant null check in amdgpu_dm.c

[Tuo Li <islituo@gmail.com>]

Hello,

Our static analysis tool has identified a potential null-pointer dereference or
redundant null check related to the wait-completion synchronization mechanism in
amdgpu_dm.c in Linux 6.11.

Consider the following execution scenario:

  dmub_aux_setconfig_callback()      //731
    if (adev->dm.dmub_notify)        //734
    complete(&adev->dm.dmub_aux_transfer_done);  //737

The variable adev->dm.dmub_notify is checked by an if statement at Line 734,
which indicates that adev->dm.dmub_notify can NULL. Then, complete() is called
at Line 737 which wakes up the wait_for_completion().

Consider the wait_for_completion()

  amdgpu_dm_process_dmub_aux_transfer_sync()    //12271
    p_notify = adev->dm.dmub_notify;            //12278
    wait_for_completion_timeout(&adev->dm.dmub_aux_transfer_done, ...); // 12287
    if (p_notify->result != AUX_RET_SUCCESS)    //12293

The value of adev->dm.dmub_notify is assigned to p_notify at Line 12278. If
adev->dm.dmub_notify at Line 734 is checked to be NULL, the value p_notify after
the wait_for_completion_timeout() at Line 12278 can also be NULL. However, it is
dereferenced at Line 12293 without rechecking, causing a possible null dereference.

In fact, dmub_aux_setconfig_callback() is registered only if
adev->dm.dmub_notify is checked to be not NULL:

  adev->dm.dmub_notify = kzalloc(...);    //2006
  if (!adev->dm.dmub_notify) {            //2007
    ......
    goto error;                           //2009
  }                                       //2010
  ......
  register_dmub_notify_callback(..., dmub_aux_setconfig_callback, ...)  //2019

I am not sure if adev->dm.dmub_notify is assigned with NULL elsewhere. If not,
the if check at Line 734 can be redundant.
Any feedback would be appreciated, thanks!

Sincerely,
Tuo Li