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Date:   Thu, 26 Aug 2021 14:51:12 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org,
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Subject: Re: [Intel-gfx] [PATCH v8 1/7] drm: fix null ptr dereference in
 drm_master_release
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        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>
 <05f4d472-7a76-598a-e792-a847a3e8516a@gmail.com>
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On Thu, Aug 26, 2021 at 07:53:58PM +0800, Desmond Cheong Zhi Xi wrote:
> 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.

Ah right, please reference that commit with a Fixes: line.
-Daniel

> 
> > > ---
> > >   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
> > > 
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch