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Subject: Re: [PATCH] drm: Lock pointer access in drm_master_release()
To:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Dan Carpenter <dan.carpenter@oracle.com>
References: <20210609092119.173590-1-desmondcheongzx@gmail.com>
 <YMHlLQHRLWgWlXTs@phenom.ffwll.local>
 <f16f4123-bd0b-f09c-ddf1-7197c841b588@gmail.com>
 <YMJCdG7k5sNaiHen@phenom.ffwll.local>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <d8150fdb-3a59-e491-f148-1c528fe3c824@gmail.com>
Date:   Fri, 11 Jun 2021 10:18:22 +0800
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On 11/6/21 12:48 am, Daniel Vetter wrote:
> On Thu, Jun 10, 2021 at 11:21:39PM +0800, Desmond Cheong Zhi Xi wrote:
>> On 10/6/21 6:10 pm, Daniel Vetter wrote:
>>> On Wed, Jun 09, 2021 at 05:21:19PM +0800, Desmond Cheong Zhi Xi wrote:
>>>> This patch eliminates the following smatch warning:
>>>> drivers/gpu/drm/drm_auth.c:320 drm_master_release() warn: unlocked access 'master' (line 318) expected lock '&dev->master_mutex'
>>>>
>>>> The 'file_priv->master' field should be protected by the mutex lock to
>>>> '&dev->master_mutex'. This is because other processes can concurrently
>>>> modify this field and free the current 'file_priv->master'
>>>> pointer. This could result in a use-after-free error when 'master' is
>>>> dereferenced in subsequent function calls to
>>>> 'drm_legacy_lock_master_cleanup()' or to 'drm_lease_revoke()'.
>>>>
>>>> An example of a scenario that would produce this error can be seen
>>>> from a similar bug in 'drm_getunique()' that was reported by Syzbot:
>>>> https://syzkaller.appspot.com/bug?id=148d2f1dfac64af52ffd27b661981a540724f803
>>>>
>>>> In the Syzbot report, another process concurrently acquired the
>>>> device's master mutex in 'drm_setmaster_ioctl()', then overwrote
>>>> 'fpriv->master' in 'drm_new_set_master()'. The old value of
>>>> 'fpriv->master' was subsequently freed before the mutex was unlocked.
>>>>
>>>> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>>>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
>>>
>>> Thanks a lot. I've done an audit of this code, and I found another
>>> potential problem in drm_is_current_master. The callers from drm_auth.c
>>> hold the dev->master_mutex, but all the external ones dont. I think we
>>> need to split this into a _locked function for use within drm_auth.c, and
>>> the exported one needs to grab the dev->master_mutex while it's checking
>>> master status. Ofc there will still be races, those are ok, but right now
>>> we run the risk of use-after free problems in drm_lease_owner.
>>>
>>> Are you up to do that fix too?
>>>
>>
>> Hi Daniel,
>>
>> Thanks for the pointer, I'm definitely up for it!
>>
>>> I think the drm_lease.c code also needs an audit, there we'd need to make
>>> sure that we hold hold either the lock or a full master reference to avoid
>>> the use-after-free issues here.
>>>
>>
>> I'd be happy to look into drm_lease.c as well.
>>
>>> Patch merged to drm-misc-fixes with cc: stable.
>>> -Daniel
>>>
>>>> ---
>>>>    drivers/gpu/drm/drm_auth.c | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>>>> index f00e5abdbbf4..b59b26a71ad5 100644
>>>> --- a/drivers/gpu/drm/drm_auth.c
>>>> +++ b/drivers/gpu/drm/drm_auth.c
>>>> @@ -315,9 +315,10 @@ int drm_master_open(struct drm_file *file_priv)
>>>>    void drm_master_release(struct drm_file *file_priv)
>>>>    {
>>>>    	struct drm_device *dev = file_priv->minor->dev;
>>>> -	struct drm_master *master = file_priv->master;
>>>> +	struct drm_master *master;
>>>>
>>>>    	mutex_lock(&dev->master_mutex);
>>>> +	master = file_priv->master;
>>>>    	if (file_priv->magic)
>>>>    		idr_remove(&file_priv->master->magic_map, file_priv->magic);
>>>> -- 
>>>> 2.25.1
>>>>
>>>
>>
>>  From what I can see, there are other places in the kernel that could use the
>> _locked version of drm_is_current_master as well, such as drm_mode_getfb in
>> drm_framebuffer.c. I'll take a closer look, and if the changes make sense
>> I'll prepare a patch series for them.
> 
> Oh maybe we have a naming confusion: the _locked is the one where the
> caller must grab the lock already, whereas drm_is_current_master would
> grab the master_mutex internally to do the check. The one in
> drm_framebuffer.c looks like it'd need the internal one since there's no
> other need to grab the master_mutex.
> -Daniel
> 

Ah ok got it, I think I confused myself earlier.

Just to check, may I include you in a Reported-by: tag?