[PATCH v2 2/2] dma-buf: Annul dmabuf->file on file release

[Matt Evans <matt@ozlabs.org>]

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  This assumption was no longer valid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: stable@vger.kernel.org
Fixes: 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops")
Signed-off-by: Matt Evans <matt@ozlabs.org>
---
 drivers/dma-buf/dma-buf.c | 21 ++++++++++++++++++---
 include/linux/dma-buf.h   |  4 +++-
 2 files changed, 21 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..9f3c92c71c29 100644
--- a/include/linux/dma-buf.h
+++ b/include/linux/dma-buf.h
@@ -303,7 +303,9 @@ 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().  Exporters 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