[PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs
Desmond Cheong Zhi Xi
desmondcheongzx at gmail.com
Mon Jul 12 04:35:03 UTC 2021
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():
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(-)
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