[PATCH v3 0/2] drm: address potential UAF bugs with drm_master ptrs

[Desmond Cheong Zhi Xi]

This patch series addresses potential use-after-free errors when dereferencing pointers to struct drm_master. These were identified after one such bug was caught by Syzbot in drm_getunique():
https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803

The series is broken up into two patches:

1. Implement a locked version of drm_is_current_master() function that's used within drm_auth.c.

2. Identify areas in drm_lease.c where pointers to struct drm_master are dereferenced, and ensure that the master pointers are not freed during use.

Changes in v2 -> v3:
- Patch 1: Move the definition of drm_is_current_master and the _locked version higher up in drm_auth.c to avoid needing a forward declaration of drm_is_current_master_locked. As suggested by Daniel Vetter.

- Patch 2: Instead of leaking drm_device.master_mutex into drm_lease.c to protect drm_master pointers, add a new drm_file_get_master() function that returns drm_file->master while increasing its reference count, to prevent drm_file->master from being freed. As suggested by Daniel Vetter.

Changes in v1 -> v2:
- Patch 2: Move the lock and assignment before the DRM_DEBUG_LEASE in drm_mode_get_lease_ioctl, as suggested by Emil Velikov.

Desmond Cheong Zhi Xi (2):
  drm: add a locked version of drm_is_current_master
  drm: protect drm_master pointers in drm_lease.c

 drivers/gpu/drm/drm_auth.c  | 73 +++++++++++++++++++++++++++----------
 drivers/gpu/drm/drm_lease.c | 57 ++++++++++++++++++++---------
 include/drm/drm_auth.h      |  1 +
 include/drm/drm_file.h      | 15 ++++++--
 4 files changed, 107 insertions(+), 39 deletions(-)

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2.25.1