[PATCH v4 0/2] drm: address potential UAF bugs with drm_master ptrs

Desmond Cheong Zhi Xi desmondcheongzx at gmail.com
Wed Jun 23 11:37:38 UTC 2021

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():

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 v3 -> v4:
- Patch 1:
Move the call to drm_is_current_master in drm_mode_getconnector out from the section locked by &dev->mode_config.mutex. As suggested by Daniel Vetter. This avoids a circular lock lock dependency as reported here https://patchwork.freedesktop.org/patch/440406/

Additionally, inside drm_is_current_master, instead of grabbing &fpriv->master->dev->master_mutex, we grab &fpriv->minor->dev->master_mutex to avoid dereferencing a null ptr if fpriv->master is not set.

- Patch 2:
Modify kerneldoc formatting.

Additionally, add a file_priv->master NULL check inside drm_file_get_master, and handle the NULL result accordingly in drm_lease.c. As suggested by Daniel Vetter.

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      | 76 ++++++++++++++++++++++++--------
 drivers/gpu/drm/drm_connector.c |  5 ++-
 drivers/gpu/drm/drm_lease.c     | 77 ++++++++++++++++++++++++---------
 include/drm/drm_auth.h          |  1 +
 include/drm/drm_file.h          | 15 +++++--
 5 files changed, 131 insertions(+), 43 deletions(-)


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