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 This assumption was no longer valid after 4ab59c3c638c6 ("dma-buf: Move dma_buf_release() from fops to dentry_ops"), which moved the callback to the dentry release (by which point the file might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 | 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;