On Wed, Jul 21, 2021 at 2:44 PM Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx> wrote: > On 21/7/21 6:29 pm, Daniel Vetter wrote: > > On Wed, Jul 21, 2021 at 6:12 AM Desmond Cheong Zhi Xi > > <desmondcheongzx@xxxxxxxxx> wrote: > >> On 21/7/21 2:24 am, Daniel Vetter wrote: > >>> 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=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. btw since we now looked at all this in detail, can you perhaps do a patch to update the kerneldoc for all the lease fields in struct drm_master? I think moving them to the inline style and then adding comments for each field how locking/lifetime rules work would be really good. Since right now it's all fresh from for us. -Daniel > >>> 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 > >>>> > >>> > >> > > > > > -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
