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

Wed Jul 21 10:29:04 UTC 2021

On Wed, Jul 21, 2021 at 6:12 AM Desmond Cheong Zhi Xi
<desmondcheongzx at 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.

> > 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