[PATCH 5.4 087/168] vt: selection, push sel_lock up

From: Jiri Slaby <redacted>

commit e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 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

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

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 <redacted>
Reported-by: <redacted>
Fixes: 07e6124a1a46 ("vt: selection, close sel_buffer race")
Cc: stable <redacted>
Link: https://lore.kernel.org/r/20200228115406.5735-2-jslaby@suse.cz (local)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/tty/vt/selection.c |   16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

--- a/drivers/tty/vt/selection.c
+++ b/drivers/tty/vt/selection.c

@@ -214,7 +214,6 @@ static int __set_selection_kernel(struct
 	if (ps > pe)	/* make sel_start <= sel_end */
 		swap(ps, pe);
 
-	mutex_lock(&sel_lock);
 	if (sel_cons != vc_cons[fg_console].d) {
 		clear_selection();
 		sel_cons = vc_cons[fg_console].d;

@@ -260,10 +259,9 @@ static int __set_selection_kernel(struct
 			break;
 		case TIOCL_SELPOINTER:
 			highlight_pointer(pe);
-			goto unlock;
+			return 0;
 		default:
-			ret = -EINVAL;
-			goto unlock;
+			return -EINVAL;
 	}
 
 	/* remove the pointer */

@@ -285,7 +283,7 @@ static int __set_selection_kernel(struct
 	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 */

@@ -313,8 +311,7 @@ static int __set_selection_kernel(struct
 	if (!bp) {
 		printk(KERN_WARNING "selection: kmalloc() failed\n");
 		clear_selection();
-		ret = -ENOMEM;
-		goto unlock;
+		return -ENOMEM;
 	}
 	kfree(sel_buffer);
 	sel_buffer = bp;

@@ -339,8 +336,7 @@ static int __set_selection_kernel(struct
 		}
 	}
 	sel_buffer_lth = bp - sel_buffer;
-unlock:
-	mutex_unlock(&sel_lock);
+
 	return ret;
 }
 

@@ -348,9 +344,11 @@ int set_selection_kernel(struct tiocl_se
 {
 	int ret;
 
+	mutex_lock(&sel_lock);
 	console_lock();
 	ret = __set_selection_kernel(v, tty);
 	console_unlock();
+	mutex_unlock(&sel_lock);
 
 	return ret;
 }