netdev - [PATCH] net: bluetooth: cmtp: fix file refcount when cmtp_attach

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Message-Id: <20210413162103.435467-1-cascardo@canonical.com>
Date:   Tue, 13 Apr 2021 13:21:03 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     linux-bluetooth@...r.kernel.org
Cc:     netdev@...r.kernel.org, kuba@...nel.org, davem@...emloft.net,
        isdn@...ux-pingi.de, marcel@...tmann.org, johan.hedberg@...il.com,
        luiz.dentz@...il.com,
        Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
Subject: [PATCH] net: bluetooth: cmtp: fix file refcount when cmtp_attach_device fails

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@...onical.com>
---
 net/bluetooth/cmtp/core.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/bluetooth/cmtp/core.c b/net/bluetooth/cmtp/core.c
index 07cfa3249f83..0a2d78e811cf 100644
--- a/net/bluetooth/cmtp/core.c
+++ b/net/bluetooth/cmtp/core.c
@@ -392,6 +392,11 @@ int cmtp_add_connection(struct cmtp_connadd_req *req, struct socket *sock)
 	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);
-- 
2.27.0