[PATCH] tty: fix pid refcount leak in tty_signal_session_leader

[Xin Xiong <xiongx18@fudan.edu.cn>]

In the loop, every time when p->signal->leader is true, the function
tty_signal_session_leader() will invoke get_pid() and return a
reference of tty->pgrp with increased refcount to the local variable
tty_pgrp or return NULL if it fails. After finishing the loop, the
function invokes put_pid() for only once, decreasing the refcount that
tty_pgrp keeps.

Refcount leaks may occur when the scenario that p->signal->leader is
true happens more than once. In this assumption, if the above scenario
happens n times in the loop, the function forgets to decrease the
refcount for n-1 times, which causes refcount leaks.

Fix the issue by decreasing the current refcount of the local variable
tty_pgrp before assigning new objects to it.

Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Xin Xiong <xiongx18@fudan.edu.cn>
---
 drivers/tty/tty_jobctrl.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index f8ed50a16848..9e6bf693ade1 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -212,6 +212,8 @@ int tty_signal_session_leader(struct tty_struct *tty, int exit_session)
 			__group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
 			put_pid(p->signal->tty_old_pgrp);  /* A noop */
 			spin_lock(&tty->ctrl_lock);
+			if (tty_pgrp)
+				put_pid(tty_pgrp);
 			tty_pgrp = get_pid(tty->pgrp);
 			if (tty->pgrp)
 				p->signal->tty_old_pgrp = get_pid(tty->pgrp);
-- 
2.25.1