Messages in this thread | | | Subject | Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs | From | Desmond Cheong Zhi Xi <> | Date | Thu, 22 Jul 2021 17:38:58 +0800 |
| |
On 21/7/21 9:23 pm, Daniel Vetter wrote: > On Wed, Jul 21, 2021 at 2:44 PM Desmond Cheong Zhi Xi > <desmondcheongzx@gmail.com> 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@gmail.com> 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 >
Sure thing. Just sent out the suggested changes in the same series, along with a relevant fix for drm/vmwgfx that I just noticed.
>>>>> 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 >>>>>> >>>>> >>>> >>> >>> >> > >
| |