linux-kernel - [PATCH 1/3] tty: Fix __tty_check

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Date:   Thu, 31 Aug 2023 18:50:29 -0700
From:   Earl Chew <earl.chew@...oo.ca>
To:     linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org,
        jirislaby@...nel.org
Cc:     peter@...leysoftware.com, earl.chew@...oo.ca
Subject: [PATCH 1/3] tty: Fix __tty_check_change() and tiocspgrp() race

Restore the use of tty->ctrl.lock to cover both checking
current->task_pgrp() and sending kill_pgrp(). This coverage
was lost for SIGTTIN as part of commit 2812d9e9fd94
(tty: Combine SIGTTOU/SIGTTIN handling). In rare cases,
job control shells using tcsetpgrp() and SIGCONT to move
a background job to the foreground could find the new
foreground job stopping almost immediately due to SIGTTIN
or SIGTTOU.

Signed-off-by: Earl Chew <earl.chew@...oo.ca>
---
 drivers/tty/tty_jobctrl.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 0d04287da..aba721a3c 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -44,7 +44,6 @@ int __tty_check_change(struct tty_struct *tty, int sig)
 
 	spin_lock_irqsave(&tty->ctrl.lock, flags);
 	tty_pgrp = tty->ctrl.pgrp;
-	spin_unlock_irqrestore(&tty->ctrl.lock, flags);
 
 	if (tty_pgrp && pgrp != tty_pgrp) {
 		if (is_ignored(sig)) {
@@ -58,6 +57,7 @@ int __tty_check_change(struct tty_struct *tty, int sig)
 			ret = -ERESTARTSYS;
 		}
 	}
+	spin_unlock_irqrestore(&tty->ctrl.lock, flags);
 	rcu_read_unlock();
 
 	if (!tty_pgrp)
-- 
2.39.1