[media] media: vb2: Fix potential deadlock in vb2_prepare_buffer

Commit b037c0fde22b1d3cd0b3c3717d28e54619fc1592 ("media: vb2: fix
potential deadlock in mmap vs. get_userptr handling") fixes an AB-BA
deadlock related to the mmap_sem and driver locks. The same deadlock can
occur in vb2_prepare_buffer(), fix it the same way.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/v4l2-core/videobuf2-core.c

@@ -1248,50 +1248,84 @@ static int __buf_prepare(struct vb2_buffer *vb, const struct v4l2_buffer *b)
  */
 int vb2_prepare_buf(struct vb2_queue *q, struct v4l2_buffer *b)
 {
+	struct rw_semaphore *mmap_sem = NULL;
 	struct vb2_buffer *vb;
 	int ret;
 
+	/*
+	 * In case of user pointer buffers vb2 allocator needs to get direct
+	 * access to userspace pages. This requires getting read access on
+	 * mmap semaphore in the current process structure. The same
+	 * semaphore is taken before calling mmap operation, while both mmap
+	 * and prepare_buf are called by the driver or v4l2 core with driver's
+	 * lock held. To avoid a AB-BA deadlock (mmap_sem then driver's lock in
+	 * mmap and driver's lock then mmap_sem in prepare_buf) the videobuf2
+	 * core release driver's lock, takes mmap_sem and then takes again
+	 * driver's lock.
+	 *
+	 * To avoid race with other vb2 calls, which might be called after
+	 * releasing driver's lock, this operation is performed at the
+	 * beggining of prepare_buf processing. This way the queue status is
+	 * consistent after getting driver's lock back.
+	 */
+	if (q->memory == V4L2_MEMORY_USERPTR) {
+		mmap_sem = &current->mm->mmap_sem;
+		call_qop(q, wait_prepare, q);
+		down_read(mmap_sem);
+		call_qop(q, wait_finish, q);
+	}
+
 	if (q->fileio) {
 		dprintk(1, "%s(): file io in progress\n", __func__);
-		return -EBUSY;
+		ret = -EBUSY;
+		goto unlock;
 	}
 
 	if (b->type != q->type) {
 		dprintk(1, "%s(): invalid buffer type\n", __func__);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	if (b->index >= q->num_buffers) {
 		dprintk(1, "%s(): buffer index out of range\n", __func__);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	vb = q->bufs[b->index];
 	if (NULL == vb) {
 		/* Should never happen */
 		dprintk(1, "%s(): buffer is NULL\n", __func__);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	if (b->memory != q->memory) {
 		dprintk(1, "%s(): invalid memory type\n", __func__);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	if (vb->state != VB2_BUF_STATE_DEQUEUED) {
 		dprintk(1, "%s(): invalid buffer state %d\n", __func__, vb->state);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 	ret = __verify_planes_array(vb, b);
 	if (ret < 0)
-		return ret;
+		goto unlock;
+
 	ret = __buf_prepare(vb, b);
 	if (ret < 0)
-		return ret;
+		goto unlock;
 
 	__fill_v4l2_buffer(vb, b);
 
-	return 0;
+unlock:
+	if (mmap_sem)
+		up_read(mmap_sem);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(vb2_prepare_buf);