linux-kernel - Re: [PATCH v2 1/2] drm: Add a locked version of drm_is_current

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Message-ID: <793cf08c-a766-637e-b473-91f6c8de12ec@gmail.com>
Date:   Fri, 18 Jun 2021 10:54:58 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        emil.l.velikov@...il.com
Subject: Re: [PATCH v2 1/2] drm: Add a locked version of drm_is_current_master

On 18/6/21 1:03 am, Daniel Vetter wrote:
> On Tue, Jun 15, 2021 at 10:36:44AM +0800, Desmond Cheong Zhi Xi wrote:
>> While checking the master status of the DRM file in
>> drm_is_current_master(), the device's master mutex should be
>> held. Without the mutex, the pointer fpriv->master may be freed
>> concurrently by another process calling drm_setmaster_ioctl(). This
>> could lead to use-after-free errors when the pointer is subsequently
>> dereferenced in drm_lease_owner().
>>
>> The callers of drm_is_current_master() from drm_auth.c hold the
>> device's master mutex, but external callers do not. Hence, we implement
>> drm_is_current_master_locked() to be used within drm_auth.c, and
>> modify drm_is_current_master() to grab the device's master mutex
>> before checking the master status.
>>
>> Reported-by: Daniel Vetter <daniel.vetter@...ll.ch>
>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
>> Reviewed-by: Emil Velikov <emil.l.velikov@...il.com>
>> ---
>>   drivers/gpu/drm/drm_auth.c | 23 +++++++++++++++++++----
>>   1 file changed, 19 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
>> index 232abbba3686..c6bf52c310a9 100644
>> --- a/drivers/gpu/drm/drm_auth.c
>> +++ b/drivers/gpu/drm/drm_auth.c
>> @@ -61,6 +61,8 @@
>>    * trusted clients.
>>    */
>>   
>> +static bool drm_is_current_master_locked(struct drm_file *fpriv);
> 
> A bit a bikeshed, but we try to avoid forward declarations when they're
> not needed. If you don't want to tear apart drm_is_current_master and the
> _locked version then just move them together.
> 
> Can you pls do that and respin?
> 
> Otherwise looks all great.
> -Daniel
> 
> 

Yeah, I was trying to keep the logic in _locked close to 
drm_is_current_master. But got it, will do.