[PATCH v2] drm/atomic-helpers: fix uaf in drm_atomic_helper_wait_for_vblanks

[Edward Adam Davis <eadavis@qq.com>]

To briefly summarize the issues reported by syzbot, there are two task call stacks
as follows:

Task A                                 Task B
------------------------------------   ----------------------------------
                                       drm_atomic_commit
                                       drm_atomic_helper_commit
commit_work
commit_tail
drm_atomic_helper_commit_tail          commit_tail 
                                       drm_atomic_helper_commit_tail
                                       drm_client_modeset_commit_atomic
                                       drm_atomic_state_default_clear
drm_atomic_helper_wait_for_vblanks

When two prerequisites are met simultaneously, the current issue will be triggered:
1. There is an overlap in the memory range occupied by the crtcs member set contained
   in the instance state of "struct drm_atomic_state" created by Task A and Task B
2. The context of drm_atomic_helper_commit_tail() has no lock protection, resulting
   in the instance state->crtcs sub item being released by other task

The solution is to add a lock in drm_atomic_helper_commit_tail() to ensure that
there is no other task interference when accessing the instance state.

Reported-and-tested-by: syzbot+0f999d26a4fd79c3a23b@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
---
 drivers/gpu/drm/drm_atomic_helper.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 39ef0a6addeb..b16ff9020097 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -1743,7 +1743,9 @@ EXPORT_SYMBOL(drm_atomic_helper_wait_for_flip_done);
 void drm_atomic_helper_commit_tail(struct drm_atomic_state *old_state)
 {
 	struct drm_device *dev = old_state->dev;
+	static DEFINE_MUTEX(lock);
 
+	mutex_lock(&lock);
 	drm_atomic_helper_commit_modeset_disables(dev, old_state);
 
 	drm_atomic_helper_commit_planes(dev, old_state, 0);
@@ -1757,6 +1759,7 @@ void drm_atomic_helper_commit_tail(struct drm_atomic_state *old_state)
 	drm_atomic_helper_wait_for_vblanks(dev, old_state);
 
 	drm_atomic_helper_cleanup_planes(dev, old_state);
+	mutex_unlock(&lock);
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_tail);
 
-- 
2.43.0