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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. >>> >>> 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. >>> >> >> Hi Daniel, >> >> Thanks for merging, more testing definitely sounds good to me. >> >>> 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? >>> >> >> I thought about this too, but I'm not sure if that's the best solution. >> >> drm_is_current_master calls drm_lease_owner which then walks up the tree >> of master lessors. The spinlock protects the master of the current drm >> file, but subsequent lessors aren't protected without holding the >> device's master mutex. > > But this isn't a fpriv->master pointer, but a master->lessor pointer. > Which should never ever be able to change (we'd have tons of uaf bugs > around drm_lease_owner otherwise). So I don't think there's anything > that dev->master_lock protects here that fpriv->master_lookup_lock > doesn't protect already? > > Or am I missing something? > > The comment in the struct drm_master says it's protected by > mode_config.idr_mutex, but that only applies to the idrs and lists I > think. > Ah you're right, I also completely forgot that lessees hold a reference to their lessor so nothing will be freed as long as the spinlock is held. I'll prepare that patch then, thanks for pointing it out. >>> 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 >>> >> >> Yup, now that the potential UAFs are addressed (hopefully), I'll take a >> closer look and propose a patch for this. > > Thanks a lot. > -Daniel > >> >> Best wishes, >> Desmond >> >>>> >>>> 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 >>>> >>> >> > >