[PATCH] drm: Lock pointer access in drm_master_release()

[Desmond Cheong Zhi Xi]

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 at oracle.com>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at 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.

Best wishes,
Desmond