From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Date: Tue, 13 Apr 2021 13:21:03 -0300
Subject: [PATCH] Bluetooth: cmtp: fix file refcount when cmtp_attach_device
 fails
Patch-mainline: v5.14-rc1
Git-commit: 3cfdf8fcaafa62a4123f92eb0f4a72650da3a479
References: bsc#1191961 CVE-2021-34981

When cmtp_attach_device fails, cmtp_add_connection returns the error value
which leads to the caller to doing fput through sockfd_put. But
cmtp_session kthread, which is stopped in this path will also call fput,
leading to a potential refcount underflow or a use-after-free.

Add a refcount before we signal the kthread to stop. The kthread will try
to grab the cmtp_session_sem mutex before doing the fput, which is held
when get_file is called, so there should be no races there.

Reported-by: Ryota Shiga
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Acked-by: Cho, Yu-Chen <acho@suse.com>
---
 net/bluetooth/cmtp/core.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/net/bluetooth/cmtp/core.c
+++ b/net/bluetooth/cmtp/core.c
@@ -391,6 +391,11 @@ int cmtp_add_connection(struct cmtp_conn
 	if (!(session->flags & BIT(CMTP_LOOPBACK))) {
 		err = cmtp_attach_device(session);
 		if (err < 0) {
+			/* Caller will call fput in case of failure, and so
+			 * will cmtp_session kthread.
+			 */
+			get_file(session->sock->file);
+
 			atomic_inc(&session->terminate);
 			wake_up_interruptible(sk_sleep(session->sock->sk));
 			up_write(&cmtp_session_sem);