V4L/DVB (6268): V4L: Fix a lock inversion in generic videobuf code

videobuf_qbuf takes q->lock, and then calls
q->ops->buf_prepare which by design in all drivers calls
videobuf_iolock which calls videobuf_dma_init_user and this
takes current->mm->mmap_sem

on the other hand if user calls mumap from other thread, sys_munmap
takes current->mm->mmap_sem and videobuf_vm_close takes q->lock

Since this can occur only for V4L2_MEMORY_MMAP buffers, take
current->mm->mmap_sem in qbuf, before q->lock, and don't take
current->mm->mmap_sem videobuf_dma_init_user for those buffers

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
http://thread.gmane.org/gmane.comp.video.video4linux/34978/focus=34981
Reviewed-by: Ricardo Cerqueira <v4l@cerqueira.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

drivers/media/video/videobuf-core.c

@@ -349,6 +349,9 @@ int videobuf_qbuf(struct videobuf_queue *q,
 
 	MAGIC_CHECK(q->int_ops->magic,MAGIC_QTYPE_OPS);
 
+	if (b->memory == V4L2_MEMORY_MMAP)
+		down_read(&current->mm->mmap_sem);
+
 	mutex_lock(&q->lock);
 	retval = -EBUSY;
 	if (q->reading) {
@@ -434,6 +437,10 @@ int videobuf_qbuf(struct videobuf_queue *q,
 
  done:
 	mutex_unlock(&q->lock);
+
+	if (b->memory == V4L2_MEMORY_MMAP)
+		up_read(&current->mm->mmap_sem);
+
 	return retval;
 }
 

drivers/media/video/videobuf-dma-sg.c

@@ -134,8 +134,8 @@ void videobuf_dma_init(struct videobuf_dmabuf *dma)
 	dma->magic = MAGIC_DMABUF;
 }
 
-int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
-			   unsigned long data, unsigned long size)
+static int videobuf_dma_init_user_locked(struct videobuf_dmabuf *dma,
+			int direction, unsigned long data, unsigned long size)
 {
 	unsigned long first,last;
 	int err, rw = 0;
@@ -160,12 +160,12 @@ int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
 
 	dma->varea = (void *) data;
 
-	down_read(&current->mm->mmap_sem);
+
 	err = get_user_pages(current,current->mm,
 			     data & PAGE_MASK, dma->nr_pages,
 			     rw == READ, 1, /* force */
 			     dma->pages, NULL);
-	up_read(&current->mm->mmap_sem);
+
 	if (err != dma->nr_pages) {
 		dma->nr_pages = (err >= 0) ? err : 0;
 		dprintk(1,"get_user_pages: err=%d [%d]\n",err,dma->nr_pages);
@@ -174,6 +174,17 @@ int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
 	return 0;
 }
 
+int videobuf_dma_init_user(struct videobuf_dmabuf *dma, int direction,
+			   unsigned long data, unsigned long size)
+{
+	int ret;
+	down_read(&current->mm->mmap_sem);
+	ret = videobuf_dma_init_user_locked(dma, direction, data, size);
+	up_read(&current->mm->mmap_sem);
+
+	return ret;
+}
+
 int videobuf_dma_init_kernel(struct videobuf_dmabuf *dma, int direction,
 			     int nr_pages)
 {
@@ -469,13 +480,24 @@ static int __videobuf_iolock (struct videobuf_queue* q,
 							pages );
 			if (0 != err)
 				return err;
-		} else {
+		} else if (vb->memory == V4L2_MEMORY_USERPTR) {
 			/* dma directly to userspace */
 			err = videobuf_dma_init_user( &mem->dma,
 						      PCI_DMA_FROMDEVICE,
 						      vb->baddr,vb->bsize );
 			if (0 != err)
 				return err;
+		} else {
+			/* NOTE: HACK: videobuf_iolock on V4L2_MEMORY_MMAP
+			buffers can only be called from videobuf_qbuf
+			we take current->mm->mmap_sem there, to prevent
+			locking inversion, so don't take it here */
+
+			err = videobuf_dma_init_user_locked(&mem->dma,
+						      PCI_DMA_FROMDEVICE,
+						      vb->baddr, vb->bsize);
+			if (0 != err)
+				return err;
 		}
 		break;
 	case V4L2_MEMORY_OVERLAY: