vt: selection, push sel_lock up

commit e8c75a3 upstream.

sel_lock cannot nest in the console lock. Thanks to syzkaller, the
kernel states firmly:

> WARNING: possible circular locking dependency detected
> 5.6.0-rc3-syzkaller #0 Not tainted
> ------------------------------------------------------
> syz-executor.4/20336 is trying to acquire lock:
> ffff8880a2e952a0 (&tty->termios_rwsem){++++}, at: tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136
>
> but task is already holding lock:
> ffffffff89462e70 (sel_lock){+.+.}, at: paste_selection+0x118/0x470 drivers/tty/vt/selection.c:374
>
> which lock already depends on the new lock.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 (sel_lock){+.+.}:
>        mutex_lock_nested+0x1b/0x30 kernel/locking/mutex.c:1118
>        set_selection_kernel+0x3b8/0x18a0 drivers/tty/vt/selection.c:217
>        set_selection_user+0x63/0x80 drivers/tty/vt/selection.c:181
>        tioclinux+0x103/0x530 drivers/tty/vt/vt.c:3050
>        vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364

This is ioctl(TIOCL_SETSEL).
Locks held on the path: console_lock -> sel_lock

> -> #1 (console_lock){+.+.}:
>        console_lock+0x46/0x70 kernel/printk/printk.c:2289
>        con_flush_chars+0x50/0x650 drivers/tty/vt/vt.c:3223
>        n_tty_write+0xeae/0x1200 drivers/tty/n_tty.c:2350
>        do_tty_write drivers/tty/tty_io.c:962 [inline]
>        tty_write+0x5a1/0x950 drivers/tty/tty_io.c:1046

This is write().
Locks held on the path: termios_rwsem -> console_lock

> -> #0 (&tty->termios_rwsem){++++}:
>        down_write+0x57/0x140 kernel/locking/rwsem.c:1534
>        tty_unthrottle+0x22/0x100 drivers/tty/tty_ioctl.c:136
>        mkiss_receive_buf+0x12aa/0x1340 drivers/net/hamradio/mkiss.c:902
>        tty_ldisc_receive_buf+0x12f/0x170 drivers/tty/tty_buffer.c:465
>        paste_selection+0x346/0x470 drivers/tty/vt/selection.c:389
>        tioclinux+0x121/0x530 drivers/tty/vt/vt.c:3055
>        vt_ioctl+0x3f1/0x3a30 drivers/tty/vt/vt_ioctl.c:364

This is ioctl(TIOCL_PASTESEL).
Locks held on the path: sel_lock -> termios_rwsem

> other info that might help us debug this:
>
> Chain exists of:
>   &tty->termios_rwsem --> console_lock --> sel_lock

Clearly. From the above, we have:
 console_lock -> sel_lock
 sel_lock -> termios_rwsem
 termios_rwsem -> console_lock

Fix this by reversing the console_lock -> sel_lock dependency in
ioctl(TIOCL_SETSEL). First, lock sel_lock, then console_lock.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: syzbot+26183d9746e62da329b8@syzkaller.appspotmail.com
Fixes: 07e6124 ("vt: selection, close sel_buffer race")
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200228115406.5735-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Jiri Slaby

drivers/tty/vt/selection.c

@@ -205,7 +205,6 @@ static int __set_selection(const struct tiocl_selection __user *sel, struct tty_
 		pe = tmp;
 	}
 
-	mutex_lock(&sel_lock);
 	if (sel_cons != vc_cons[fg_console].d) {
 		clear_selection();
 		sel_cons = vc_cons[fg_console].d;
@@ -251,10 +250,9 @@ static int __set_selection(const struct tiocl_selection __user *sel, struct tty_
 			break;
 		case TIOCL_SELPOINTER:
 			highlight_pointer(pe);
-			goto unlock;
+			return 0;
 		default:
-			ret = -EINVAL;
-			goto unlock;
+			return -EINVAL;
 	}
 
 	/* remove the pointer */
@@ -276,7 +274,7 @@ static int __set_selection(const struct tiocl_selection __user *sel, struct tty_
 	else if (new_sel_start == sel_start)
 	{
 		if (new_sel_end == sel_end)	/* no action required */
-			goto unlock;
+			return 0;
 		else if (new_sel_end > sel_end)	/* extend to right */
 			highlight(sel_end + 2, new_sel_end);
 		else				/* contract from right */
@@ -303,8 +301,7 @@ static int __set_selection(const struct tiocl_selection __user *sel, struct tty_
 	if (!bp) {
 		printk(KERN_WARNING "selection: kmalloc() failed\n");
 		clear_selection();
-		ret = -ENOMEM;
-		goto unlock;
+		return -ENOMEM;
 	}
 	kfree(sel_buffer);
 	sel_buffer = bp;
@@ -329,18 +326,19 @@ static int __set_selection(const struct tiocl_selection __user *sel, struct tty_
 		}
 	}
 	sel_buffer_lth = bp - sel_buffer;
-unlock:
-	mutex_unlock(&sel_lock);
+
 	return ret;
 }
 
 int set_selection(const struct tiocl_selection __user *v, struct tty_struct *tty)
 {
 	int ret;
 
+	mutex_lock(&sel_lock);
 	console_lock();
 	ret = __set_selection(v, tty);
 	console_unlock();
+	mutex_unlock(&sel_lock);
 
 	return ret;
 }