[media] media: vb2: fix potential deadlock in mmap vs. get_userptr handling

To get direct access to userspace memory pages vb2 allocator needs to
gather read access on mmap semaphore in the current process.
The same semaphore is taken before calling mmap operation, while
both mmap and qbuf 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 qbuf)
the videobuf2 core release driver's lock, takes mmap_sem and then
takes again driver's lock. get_userptr methods are now called with
all needed locks already taken to avoid further lock magic inside
memory allocator's code.

Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 42 insertions, 9 deletions

diff --git a/drivers/media/video/videobuf2-core.c b/drivers/media/video/videobuf2-core.c
index 4d22b8214f98..a3a9bf908d84 100644
--- a/drivers/media/video/videobuf2-core.c
+++ b/drivers/media/video/videobuf2-core.c
@@ -1082,46 +1082,76 @@ EXPORT_SYMBOL_GPL(vb2_prepare_buf);
  */
 int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
 {
+	struct rw_semaphore *mmap_sem = NULL;
 	struct vb2_buffer *vb;
-	int ret;
+	int ret = 0;
+
+	/*
+	 * 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 qbuf 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 qbuf) 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 qbuf processing. This way the queue status is
+	 * consistent after getting driver's lock back.
+	 */
+	if (b->type == 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, "qbuf: file io in progress\n");
-		return -EBUSY;
+		ret = -EBUSY;
+		goto unlock;
 	}
 
 	if (b->type != q->type) {
 		dprintk(1, "qbuf: invalid buffer type\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	if (b->index >= q->num_buffers) {
 		dprintk(1, "qbuf: buffer index out of range\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	vb = q->bufs[b->index];
 	if (NULL == vb) {
 		/* Should never happen */
 		dprintk(1, "qbuf: buffer is NULL\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	if (b->memory != q->memory) {
 		dprintk(1, "qbuf: invalid memory type\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	switch (vb->state) {
 	case VB2_BUF_STATE_DEQUEUED:
 		ret = __buf_prepare(vb, b);
 		if (ret)
-			return ret;
+			goto unlock;
 	case VB2_BUF_STATE_PREPARED:
 		break;
 	default:
 		dprintk(1, "qbuf: buffer already in use\n");
-		return -EINVAL;
+		ret = -EINVAL;
+		goto unlock;
 	}
 
 	/*
@@ -1142,7 +1172,10 @@ int vb2_qbuf(struct vb2_queue *q, struct v4l2_buffer *b)
 	__fill_v4l2_buffer(vb, b);
 
 	dprintk(1, "qbuf of buffer %d succeeded\n", vb->v4l2_buf.index);
-	return 0;
+unlock:
+	if (mmap_sem)
+		up_read(mmap_sem);
+	return ret;
 }
 EXPORT_SYMBOL_GPL(vb2_qbuf);