The dmabuf release path is split between file and dentry release:
dma_buf_file_release() is called shortly before the file is freed, and
dma_buf_release() calls an exporter's dmabuf->ops->release when the
dentry is freed. However, the dentry can outlive the file (for
example, if opened with O_PATH), meaning .release might be called some
time after the file is freed.
This presents a window, when closing a DMABUF file, in which
dmabuf->file points to freed memory yet .release op has not yet been
called.
For VFIO, if a buffer's .release has not been called it's considered
still active and subject to move/cleanup. If so, it attempts to
get_file_active() on dmabuf->file: a file close with dentry held open
will call this function with a stale pointer, a UAF.
To make this pattern safe, set dmabuf->file to NULL in
dma_buf_file_release() to reflect that the associated file is now dead
even if the DMABUF is not yet gone. A get_file_active() ... fput()
sequence concurrent with a file close then only executes as one of:
- Gets the file before file_ref_put() (dmabuf->file valid)
- Observes dmabuf->file pointing to a file, but it's DEAD (no file)
- Observes dmabuf->file = NULL (no file)
Originally, drivers could assume dmabuf->file was valid until .release
was called from fops->release. This assumption was no longer valid
after 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to
dentry_ops"), which moved the callback to the dentry release (by which
point the file might have been freed). With this commit, drivers must
still consider that dmabuf->file could be NULL before .release.
Cc: [email protected]
Fixes: 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
Signed-off-by: Matt Evans <[email protected]>
---
drivers/dma-buf/dma-buf.c | 21 ++++++++++++++++++---
include/linux/dma-buf.h | 5 ++++-
2 files changed, 22 insertions(+), 4 deletions(-)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 4c9add51f9ef..61485fc28b8b 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -193,11 +193,26 @@ static void dma_buf_release(struct dentry *dentry)
static int dma_buf_file_release(struct inode *inode, struct file *file)
{
- if (!is_dma_buf_file(file))
- return -EINVAL;
+ struct dma_buf *dmabuf = file->private_data;
+
+ if (!dmabuf)
+ return 0;
- __dma_buf_list_del(file->private_data);
+ __dma_buf_list_del(dmabuf);
+ /*
+ * As the file is being closed, dmabuf->file is no longer
+ * valid and NULL replaces an otherwise stale pointer. If a
+ * reference to the file isn't already held, the file can be
+ * closed at any time, including before dmabuf->ops->release()
+ * happens: dmabuf->file _must not_ be accessed directly. An
+ * exporter should use get_file_active(&dmabuf->file) which
+ * atomically tests and acquires a reference to the file (or
+ * returns NULL if the file has closed). The barrier ensures
+ * get_file_active() observes the store before file_free()
+ * frees the memory.
+ */
+ smp_store_mb(dmabuf->file, NULL);
return 0;
}
diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
index d15b2b31d3c9..a31eb0ab9ecf 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -303,7 +303,10 @@ struct dma_buf {
* @file:
*
* File pointer used for sharing buffers across, and for refcounting.
- * See dma_buf_get() and dma_buf_put().
+ * See dma_buf_get() and dma_buf_put(). When an exporter holds a
+ * common lock with the ->release() callback, it can use
+ * get_file_active(&dma_buf->file) to safely acquire a reference;
+ * the file could otherwise be closed at any time.
*/
struct file *file;
--
2.47.3
