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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: a240d6e42e ("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>
This commit is contained in:
Al Viro 2025-07-12 06:09:16 +01:00 committed by Juergen Gross
parent 0df1195009
commit 532c8b51b3
1 changed files with 10 additions and 18 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
drivers/xen/gntdev-dmabuf.c
View File

@ -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 *