From 54ffccbf053b5b6ca4f6e45094b942fab92a25fc Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Thu, 3 Dec 2020 02:25:04 +0100
Subject: [PATCH] tty: Fix ->pgrp locking in tiocspgrp()
Git-commit: 54ffccbf053b5b6ca4f6e45094b942fab92a25fc
Patch-mainline: v5.10-rc7
References: bsc#1179745 CVE-2020-29661

tiocspgrp() takes two tty_struct pointers: One to the tty that userspace
passed to ioctl() (`tty`) and one to the TTY being changed (`real_tty`).
These pointers are different when ioctl() is called with a master fd.

To properly lock real_tty->pgrp, we must take real_tty->ctrl_lock.

This bug makes it possible for racing ioctl(TIOCSPGRP, ...) calls on
both sides of a PTY pair to corrupt the refcount of `struct pid`,
leading to use-after-free errors.

Fixes: 47f86834bbd4 ("redo locking of tty->pgrp")
Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/tty/tty_jobctrl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 28a23a0fef21..baadeea4a289 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -494,10 +494,10 @@ static int tiocspgrp(struct tty_struct *tty, struct tty_struct *real_tty, pid_t
 	if (session_of_pgrp(pgrp) != task_session(current))
 		goto out_unlock;
 	retval = 0;
-	spin_lock_irq(&tty->ctrl_lock);
+	spin_lock_irq(&real_tty->ctrl_lock);
 	put_pid(real_tty->pgrp);
 	real_tty->pgrp = get_pid(pgrp);
-	spin_unlock_irq(&tty->ctrl_lock);
+	spin_unlock_irq(&real_tty->ctrl_lock);
 out_unlock:
 	rcu_read_unlock();
 	return retval;
-- 
2.26.2