habanalabs: fix UAF in export_dmabuf()

As soon as we'd inserted a file reference into descriptor table, another
thread could close it. That's fine for the case when all we are doing is
returning that descriptor to userland (it's a race, but it's a userland
race and there's nothing the kernel can do about it). However, if we
follow fd_install() with any kind of access to objects that would be
destroyed on close (be it the struct file itself or anything destroyed
by its ->release()), we have a UAF.

dma_buf_fd() is a combination of reserving a descriptor and fd_install().
habanalabs export_dmabuf() calls it and then proceeds to access the
objects destroyed on close. In particular, it grabs an extra reference to
another struct file that will be dropped as part of ->release() for ours;
that "will be" is actually "might have already been".

Fix that by reserving descriptor before anything else and do fd_install()
only when everything had been set up. As a side benefit, we no longer
have the failure exit with file already created, but reference to
underlying file (as well as ->dmabuf_export_cnt, etc.) not grabbed yet;
unlike dma_buf_fd(), fd_install() can't fail.

Fixes: db1a8dd916aa ("habanalabs: add support for dma-buf exporter")
Signed-off-by: Al Viro <[e-mail]>

drivers/accel/habanalabs/common/memory.c

···
 	struct hl_dmabuf_priv *hl_dmabuf = dmabuf->priv;
 	struct hl_ctx *ctx;
 
-	if (!hl_dmabuf)
-		return;
-
 	ctx = hl_dmabuf->ctx;
 
 	if (hl_dmabuf->memhash_hnode)
···
 {
 	DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
 	struct hl_device *hdev = ctx->hdev;
-	int rc, fd;
+	CLASS(get_unused_fd, fd)(flags);
+
+	if (fd < 0) {
+		dev_err(hdev->dev, "failed to get a file descriptor for a dma-buf, %d\n", fd);
+		return fd;
+	}
 
 	exp_info.ops = &habanalabs_dmabuf_ops;
 	exp_info.size = total_size;
···
 	if (IS_ERR(hl_dmabuf->dmabuf)) {
 		dev_err(hdev->dev, "failed to export dma-buf\n");
 		return PTR_ERR(hl_dmabuf->dmabuf);
-	}
-
-	fd = dma_buf_fd(hl_dmabuf->dmabuf, flags);
-	if (fd < 0) {
-		dev_err(hdev->dev, "failed to get a file descriptor for a dma-buf, %d\n", fd);
-		rc = fd;
-		goto err_dma_buf_put;
 	}
 
 	hl_dmabuf->ctx = ctx;
···
 	get_file(ctx->hpriv->file_priv->filp);
 
 	*dmabuf_fd = fd;
+	fd_install(take_fd(fd), hl_dmabuf->dmabuf->file);
 
 	return 0;
-
-err_dma_buf_put:
-	hl_dmabuf->dmabuf->priv = NULL;
-	dma_buf_put(hl_dmabuf->dmabuf);
-	return rc;
 }
 
 static int validate_export_params_common(struct hl_device *hdev, u64 addr, u64 size, u64 offset)