On Mon, Jul 12, 2021 at 12:35:03PM +0800, Desmond Cheong Zhi Xi wrote:

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=148d2f1dfac64af52ffd27b661981a540724f80...

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.

Apologies for the delay, I missed your series. Maybe just ping next time around there's silence.

Looks all great, merged to drm-misc-next. Given how complex this was I'm vary of just pushing this to -fixes without some solid testing.

One thing I noticed is that drm_is_current_master could just use the spinlock, since it's only doing a read access. Care to type up that patch?

Also, do you plan to look into that idea we've discussed to flush pending access when we revoke a master or a lease? I think that would be really nice improvement here. -Daniel

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