binder: fix use-after-free due to ksys_close() during fdget()

44d8047f1d8 ("binder: use standard functions to allocate fds")
exposed a pre-existing issue in the binder driver.

fdget() is used in ksys_ioctl() as a performance optimization.
One of the rules associated with fdget() is that ksys_close() must
not be called between the fdget() and the fdput(). There is a case
where this requirement is not met in the binder driver which results
in the reference count dropping to 0 when the device is still in
use. This can result in use-after-free or other issues.

If userpace has passed a file-descriptor for the binder driver using
a BINDER_TYPE_FDA object, then kys_close() is called on it when
handling a binder_ioctl(BC_FREE_BUFFER) command. This violates
the assumptions for using fdget().

The problem is fixed by deferring the close using task_work_add(). A
new variant of __close_fd() was created that returns a struct file
with a reference. The fput() is deferred instead of using ksys_close().

Fixes: 44d8047f1d87a ("binder: use standard functions to allocate fds")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Todd Kjos <tkjos@google.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 29 insertions, 0 deletions

diff --git a/fs/file.c b/fs/file.c
index 7ffd6e9d103d..8d059d8973e9 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -640,6 +640,35 @@ out_unlock:
 }
 EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
 
+/*
+ * variant of __close_fd that gets a ref on the file for later fput
+ */
+int __close_fd_get_file(unsigned int fd, struct file **res)
+{
+	struct files_struct *files = current->files;
+	struct file *file;
+	struct fdtable *fdt;
+
+	spin_lock(&files->file_lock);
+	fdt = files_fdtable(files);
+	if (fd >= fdt->max_fds)
+		goto out_unlock;
+	file = fdt->fd[fd];
+	if (!file)
+		goto out_unlock;
+	rcu_assign_pointer(fdt->fd[fd], NULL);
+	__put_unused_fd(files, fd);
+	spin_unlock(&files->file_lock);
+	get_file(file);
+	*res = file;
+	return filp_close(file, files);
+
+out_unlock:
+	spin_unlock(&files->file_lock);
+	*res = NULL;
+	return -ENOENT;
+}
+
 void do_close_on_exec(struct files_struct *files)
 {
 	unsigned i;