Date:   Tue, 20 Jul 2021 20:24:31 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
Cc:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        sumit.semwal@...aro.org, christian.koenig@....com,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        emil.l.velikov@...il.com
Subject: Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master
 ptrs

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.

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?

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

> 
> 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

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