xen: fix UAF in dmabuf_exp_from_pages()

[dma_buf_fd() fixes; no preferences regarding the tree it goes through -
up to xen folks]

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().
gntdev dmabuf_exp_from_pages() calls it and then proceeds to access the
objects destroyed on close - starting with gntdev_dmabuf itself.

Fix that by doing reserving descriptor before anything else and do
fd_install() only when everything had been set up.

Fixes: a240d6e42e28 ("xen/gntdev: Implement dma-buf export functionality")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250712050916.GY1880847@ZenIV>
Signed-off-by: Juergen Gross <jgross@suse.com>

diff --git a/drivers/xen/gntdev-dmabuf.c b/drivers/xen/gntdev-dmabuf.c
index 5453d86324f66f956b16fdb2fc7c3366ca3a75e3..82855105ab857f16c232ac5bb07cf18ff7ed996b 100644 (file)
--- a/drivers/xen/gntdev-dmabuf.c
+++ b/drivers/xen/gntdev-dmabuf.c
@@ -357,8 +357,11 @@ struct gntdev_dmabuf_export_args {
 static int dmabuf_exp_from_pages(struct gntdev_dmabuf_export_args *args)
 {
        DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
-       struct gntdev_dmabuf *gntdev_dmabuf;
-       int ret;
+       struct gntdev_dmabuf *gntdev_dmabuf __free(kfree) = NULL;
+       CLASS(get_unused_fd, ret)(O_CLOEXEC);
+
+       if (ret < 0)
+               return ret;
 
        gntdev_dmabuf = kzalloc(sizeof(*gntdev_dmabuf), GFP_KERNEL);
        if (!gntdev_dmabuf)
@@ -383,32 +386,21 @@ static int dmabuf_exp_from_pages(struct gntdev_dmabuf_export_args *args)
        exp_info.priv = gntdev_dmabuf;
 
        gntdev_dmabuf->dmabuf = dma_buf_export(&exp_info);
-       if (IS_ERR(gntdev_dmabuf->dmabuf)) {
-               ret = PTR_ERR(gntdev_dmabuf->dmabuf);
-               gntdev_dmabuf->dmabuf = NULL;
-               goto fail;
-       }
-
-       ret = dma_buf_fd(gntdev_dmabuf->dmabuf, O_CLOEXEC);
-       if (ret < 0)
-               goto fail;
+       if (IS_ERR(gntdev_dmabuf->dmabuf))
+               return PTR_ERR(gntdev_dmabuf->dmabuf);
 
        gntdev_dmabuf->fd = ret;
        args->fd = ret;
 
        pr_debug("Exporting DMA buffer with fd %d\n", ret);
 
+       get_file(gntdev_dmabuf->priv->filp);
        mutex_lock(&args->dmabuf_priv->lock);
        list_add(&gntdev_dmabuf->next, &args->dmabuf_priv->exp_list);
        mutex_unlock(&args->dmabuf_priv->lock);
-       get_file(gntdev_dmabuf->priv->filp);
-       return 0;
 
-fail:
-       if (gntdev_dmabuf->dmabuf)
-               dma_buf_put(gntdev_dmabuf->dmabuf);
-       kfree(gntdev_dmabuf);
-       return ret;
+       fd_install(take_fd(ret), no_free_ptr(gntdev_dmabuf)->dmabuf->file);
+       return 0;
 }
 
 static struct gntdev_grant_map *