[PATCH v9 0/4] drm: update locking for modesetting

Desmond Cheong Zhi Xi desmondcheongzx at gmail.com
Tue Aug 31 06:13:44 UTC 2021


I updated the patch set with some suggestions by Daniel Vetter, and
dropped the patches after patch 4 so that we can stick the landing for
avoiding races with modesetting rights before dealing with the tricky

Overall, this series fixes races with modesetting rights, and converts
drm_device.master_mutex into master_rwsem.

- Patch 1: Fix a potential null ptr dereference in drm_master_release

- Patch 2: Convert master_mutex into rwsem (avoids creating a new lock)

- Patch 3: Update global mutex locking in the ioctl handler (avoids
deadlock when grabbing read lock on master_rwsem in drm_ioctl_kernel)

- Patch 4: Plug races with drm modesetting rights

v8 -> v9 (suggested by Daniel Vetter):
- Dropped patches 5-7 to handle it in another series
- Added the appropriate Fixes: tag for the null ptr dereference fix
(patch 1)
- Create a locked_ioctl bool to clarify locking/unlocking patterns in
the ioctl handler (patch 3)
- Clarified the kernel doc for master_rwsem (patch 4)

v7 -> v8:
- Avoid calling drm_lease_held in drm_mode_setcrtc and
drm_wait_vblank_ioctl, caught by Intel-gfx CI

v6 -> v7:
- Export __drm_mode_object_find for loadable modules, caught by the
Intel-gfx CI

v5 -> v6:
- Fix recursive locking on master_rwsem, caught by the Intel-gfx CI

v4 -> v5:
- Avoid calling drm_file_get_master while holding on to the modeset
mutex, caught by the Intel-gfx CI

v3 -> v4 (suggested by Daniel Vetter):
- Drop a patch that added an unnecessary master_lookup_lock in
- Drop a patch that addressed a non-existent race in
- Remove fixes for non-existent null ptr dereferences
- Protect drm_master.magic_map,unique{_len} with master_rwsem instead of
- Drop the patch that moved master_lookup_lock into struct drm_device
- Drop a patch to export task_work_add
- Revert the check for the global mutex in the ioctl handler to use
drm_core_check_feature instead of drm_dev_needs_global_mutex
- Push down master_rwsem locking for selected ioctls to avoid lock
hierarchy inversions, and to allow us to hold write locks on
master_rwsem instead of flushing readers
- Remove master_lookup_lock by replacing it with master_rwsem

v2 -> v3:
- Unexport drm_master_flush, as suggested by Daniel Vetter.
- Merge master_mutex and master_rwsem, as suggested by Daniel Vetter.
- Export task_work_add, reported by kernel test robot.
- Make master_flush static, reported by kernel test robot.
- Move master_lookup_lock into struct drm_device.
- Add a missing lock on master_lookup_lock in drm_master_release.
- Fix a potential race in drm_is_current_master_locked.
- Fix potential null ptr dereferences in drm_{auth, ioctl}.
- Protect magic_map,unique{_len} with  master_lookup_lock.
- Convert master_mutex into a rwsem.
- Update global mutex locking in the ioctl handler.

v1 -> v2 (suggested by Daniel Vetter):
- Address an additional race when drm_open runs.
- Switch from SRCU to rwsem to synchronise readers and writers.
- Implement drm_master_flush with task_work so that flushes can be
queued to run before returning to userspace without creating a new
DRM_MASTER_FLUSH ioctl flag.

Best wishes,

Desmond Cheong Zhi Xi (4):
  drm: fix null ptr dereference in drm_master_release
  drm: convert drm_device.master_mutex into a rwsem
  drm: lock drm_global_mutex earlier in the ioctl handler
  drm: avoid races with modesetting rights

 drivers/gpu/drm/drm_auth.c    | 39 ++++++++++++++++------------
 drivers/gpu/drm/drm_debugfs.c |  4 +--
 drivers/gpu/drm/drm_drv.c     |  3 +--
 drivers/gpu/drm/drm_file.c    |  6 ++---
 drivers/gpu/drm/drm_ioctl.c   | 49 ++++++++++++++++++++++-------------
 drivers/gpu/drm/drm_lease.c   | 35 +++++++++++++++++--------
 include/drm/drm_auth.h        |  6 ++---
 include/drm/drm_device.h      | 16 +++++++++---
 include/drm/drm_file.h        | 12 ++++-----
 9 files changed, 104 insertions(+), 66 deletions(-)


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