From c336a5ee984708db4826ef9e47d184e638e29717 Mon Sep 17 00:00:00 2001
From: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Date: Wed, 9 Jun 2021 17:21:19 +0800
Subject: [PATCH] drm: Lock pointer access in drm_master_release()
Git-commit: c336a5ee984708db4826ef9e47d184e638e29717
Patch-mainline: v5.13-rc6
References: git-fixes

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@oracle.com>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210609092119.173590-1-desmondcheongzx@gmail.com
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 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 f2d46b7ac6f9..232abbba3686 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -314,9 +314,10 @@ 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);
 
-- 
2.26.2