media: vb2: vb2_mmap: move lock up

If a filehandle is dup()ped, then it is possible to close it from one fd
and call mmap from the other. This creates a race condition in vb2_mmap
where it is using queue data that __vb2_queue_free (called from close())
is in the process of releasing.

By moving up the mutex_lock(mmap_lock) in vb2_mmap this race is avoided
since __vb2_queue_free is called with the same mutex locked. So vb2_mmap
now reads consistent buffer data.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Reported-by: syzbot+be93025dd45dccd8923c@syzkaller.appspotmail.com
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/common/videobuf2/videobuf2-core.c

@@ -2117,9 +2117,13 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
 			return -EINVAL;
 		}
 	}
+
+	mutex_lock(&q->mmap_lock);
+
 	if (vb2_fileio_is_active(q)) {
 		dprintk(1, "mmap: file io in progress\n");
-		return -EBUSY;
+		ret = -EBUSY;
+		goto unlock;
 	}
 
 	/*
@@ -2127,7 +2131,7 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
 	 */
 	ret = __find_plane_by_offset(q, off, &buffer, &plane);
 	if (ret)
-		return ret;
+		goto unlock;
 
 	vb = q->bufs[buffer];
 
@@ -2143,8 +2147,9 @@ int vb2_mmap(struct vb2_queue *q, struct vm_area_struct *vma)
 		return -EINVAL;
 	}
 
-	mutex_lock(&q->mmap_lock);
 	ret = call_memop(vb, mmap, vb->planes[plane].mem_priv, vma);
+
+unlock:
 	mutex_unlock(&q->mmap_lock);
 	if (ret)
 		return ret;