[PATCH] net: bluetooth: cmtp: fix file refcount when cmtp_attach_device fails

* [PATCH] net: bluetooth: cmtp: fix file refcount when cmtp_attach_device fails
@ 2021-04-13 16:21 Thadeu Lima de Souza Cascardo
  2021-04-13 17:50 ` bluez.test.bot
  2021-04-13 18:40 ` [PATCH] " Marcel Holtmann
  0 siblings, 2 replies; 3+ messages in thread
From: Thadeu Lima de Souza Cascardo @ 2021-04-13 16:21 UTC (permalink / raw)
  To: linux-bluetooth
  Cc: netdev, kuba, davem, isdn, marcel, johan.hedberg, luiz.dentz,
	Thadeu Lima de Souza Cascardo

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>
---
 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


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