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:

quoted

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 <redacted>

Reviewed-by: Daniel Vetter <redacted>

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.

quoted

---
  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

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