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Subject: Re: [PATCH v8 1/7] drm: fix null ptr dereference in
 drm_master_release
To:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
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References: <20210826020122.1488002-1-desmondcheongzx@gmail.com>
 <20210826020122.1488002-2-desmondcheongzx@gmail.com>
 <YSdkjvWN9RAijZJy@phenom.ffwll.local>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <05f4d472-7a76-598a-e792-a847a3e8516a@gmail.com>
Date:   Thu, 26 Aug 2021 19:53:58 +0800
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On 26/8/21 5:53 pm, Daniel Vetter wrote:
> On Thu, Aug 26, 2021 at 10:01:16AM +0800, Desmond Cheong Zhi Xi wrote:
>> drm_master_release can be called on a drm_file without a master, which
>> results in a null ptr dereference of file_priv->master->magic_map. The
>> three cases are:
>>
>> 1. Error path in drm_open_helper
>>    drm_open():
>>      drm_open_helper():
>>        drm_master_open():
>>          drm_new_set_master(); <--- returns -ENOMEM,
>>                                     drm_file.master not set
>>        drm_file_free():
>>          drm_master_release(); <--- NULL ptr dereference
>>                                     (file_priv->master->magic_map)
>>
>> 2. Error path in mock_drm_getfile
>>    mock_drm_getfile():
>>      anon_inode_getfile(); <--- returns error, drm_file.master not set
>>      drm_file_free():
>>        drm_master_release(); <--- NULL ptr dereference
>>                                   (file_priv->master->magic_map)
>>
>> 3. In drm_client_close, as drm_client_open doesn't set up a master
>>
>> drm_file.master is set up in drm_open_helper through the call to
>> drm_master_open, so we mirror it with a call to drm_master_release in
>> drm_close_helper, and remove drm_master_release from drm_file_free to
>> avoid the null ptr dereference.
>>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 
> I guess we should also have a cc: stable on this one? I think this bug
> existed since pretty much forever, but maybe more prominent with the
> drm_client stuff added a while ago.
> -Daniel
> 

Thanks for the reviews, Daniel.

Took a closer look. I think if we cc: stable, this fix should accompany 
commit 7eeaeb90a6a5 ("drm/file: Don't set master on in-kernel clients") 
which moves the drm_master_open out from drm_file_alloc into 
drm_open_helper.

>> ---
>>   drivers/gpu/drm/drm_file.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_file.c b/drivers/gpu/drm/drm_file.c
>> index ed25168619fc..90b62f360da1 100644
>> --- a/drivers/gpu/drm/drm_file.c
>> +++ b/drivers/gpu/drm/drm_file.c
>> @@ -282,9 +282,6 @@ void drm_file_free(struct drm_file *file)
>>   
>>   	drm_legacy_ctxbitmap_flush(dev, file);
>>   
>> -	if (drm_is_primary_client(file))
>> -		drm_master_release(file);
>> -
>>   	if (dev->driver->postclose)
>>   		dev->driver->postclose(dev, file);
>>   
>> @@ -305,6 +302,9 @@ static void drm_close_helper(struct file *filp)
>>   	list_del(&file_priv->lhead);
>>   	mutex_unlock(&dev->filelist_mutex);
>>   
>> +	if (drm_is_primary_client(file_priv))
>> +		drm_master_release(file_priv);
>> +
>>   	drm_file_free(file_priv);
>>   }
>>   
>> -- 
>> 2.25.1
>>
>