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Subject: Re: [PATCH v8 0/5] drm: address potential UAF bugs with drm_master
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To:     Daniel Vetter <daniel@ffwll.ch>
Cc:     Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Dave Airlie <airlied@linux.ie>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        "moderated list:DMA BUFFER SHARING FRAMEWORK" 
        <linaro-mm-sig@lists.linaro.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Emil Velikov <emil.l.velikov@gmail.com>
References: <20210712043508.11584-1-desmondcheongzx@gmail.com>
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 <50c5582b-c674-4ef8-585f-7a3d78a49f85@gmail.com>
 <CAKMK7uGvb3O9Ypd73xZf6bdMcXJyGJw4C7GXGprkZLpN9Gx7qQ@mail.gmail.com>
 <52c4207a-6830-01c9-a28c-635c68de3e14@gmail.com>
 <CAKMK7uF4Y+WbiXWyvn1em4=xZAFvBusRhxYg9ue5TE003tovbA@mail.gmail.com>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <d551e389-77e5-94d4-7234-c421d99d8bf9@gmail.com>
Date:   Thu, 22 Jul 2021 17:38:58 +0800
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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
>>>>>>
>>>>>
>>>>
>>>
>>>
>>
> 
>