[v5,0/3] drm: address potential UAF bugs with drm_master ptrs

Message

Desmond Cheong Zhi Xi June 29, 2021, 3:37 a.m. UTC

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 three patches:

1. Move a call to drm_is_current_master() out from a section locked by &dev->mode_config.mutex in drm_mode_getconnector(). This patch does not apply to stable.

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

3. 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 v4 -> v5:
- Patch 1:
Add patch 1 to the series. The changes in patch 1 do not apply to stable because they apply to new changes in the drm-misc-next branch. This patch moves the call to drm_is_current_master in drm_mode_getconnector out from the section locked by &dev->mode_config.mutex.

Additionally, added a missing semicolon to the patch, caught by the intel-gfx CI.

- Patch 2:
Move changes to drm_connector.c into patch 1.

Changes in v3 -> v4:
- Patch 2:
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 3:
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 2:
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 3:
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 3:
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 (3):
  drm: avoid circular locks in drm_mode_getconnector
  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(-)