[PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

Hi,

In the previous thread on this series we decided to remove a patch that was violating a lockdep requirement in drm_lease. In addition to this change, I took a closer look at the CI logs for the Basic Acceptance Tests and noticed that another regression was introduced. The new patch 2 is a response to this.

Overall, this 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 five 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. Move a call to drm_is_current_master() out from the RCU read-side critical section in drm_clients_info().

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

4. Serialize drm_file.master by introducing a new spinlock that's held whenever the value of drm_file.master changes.

5. 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.

v7 -> v8:
- Remove the patch that moves the call to _drm_lease_held out from the section locked by &dev->mode_config.idr_mutex in __drm_mode_object_find. This patch violated an existing lockdep requirement as reported by the intel-gfx CI.
- Added a new patch that moves a call to drm_is_current_master out from the RCU critical section in drm_clients_info. This was reported by the intel-gfx CI.

v6 -> v7:
- Modify code alignment as suggested by the intel-gfx CI.
- Add a new patch to the series that adds a new lock to serialize drm_file.master, in response to the lockdep splat by the intel-gfx CI.
- Update drm_file_get_master to use the new drm_file.master_lock instead of drm_device.master_mutex, in response to the lockdep splat by the intel-gfx CI.

v5 -> v6:
- Add a new patch to the series that moves the call to _drm_lease_held out from the section locked by &dev->mode_config.idr_mutex in __drm_mode_object_find.
- Clarify the kerneldoc for dereferencing drm_file.master, as suggested by Daniel Vetter.
- Refactor error paths with goto labels so that each function only has a single drm_master_put(), as suggested by Emil Velikov.
- Modify comparisons to NULL into "!master", as suggested by the intel-gfx CI.

v4 -> v5:
- Add a new patch to the series that 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.

v3 -> v4:
- 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/
- 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.
- Modify kerneldoc formatting for drm_file.master, as suggested by Daniel Vetter.
- 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.

v2 -> v3:
- 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.
- 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.

v1 -> v2:
- 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 (5):
  drm: avoid circular locks in drm_mode_getconnector
  drm: avoid blocking in drm_clients_info's rcu section
  drm: add a locked version of drm_is_current_master
  drm: serialize drm_file.master with a new spinlock
  drm: protect drm_master pointers in drm_lease.c

 drivers/gpu/drm/drm_auth.c      | 93 ++++++++++++++++++++++++---------
 drivers/gpu/drm/drm_connector.c |  5 +-
 drivers/gpu/drm/drm_debugfs.c   |  3 +-
 drivers/gpu/drm/drm_file.c      |  1 +
 drivers/gpu/drm/drm_lease.c     | 81 +++++++++++++++++++++-------
 include/drm/drm_auth.h          |  1 +
 include/drm/drm_file.h          | 18 +++++--
 7 files changed, 152 insertions(+), 50 deletions(-)

-- 
2.25.1