Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master ptrs

[Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>]

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.

>>> 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
>>>>
>>>
>>
> 
>