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patch "vt: selection, push sel_lock up" added to tty-linus

PATCH [email protected] 1 participant 1 message Feb 28, 2020

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This is a note to let you know that I've just added the patch titled

vt: selection, push sel_lock up

to my tty git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git
in the tty-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From e8c75a30a23c6ba63f4ef6895cbf41fd42f21aa2 Mon Sep 17 00:00:00 2001
From: Jiri Slaby <[email protected]>
Date: Fri, 28 Feb 2020 12:54:06 +0100
Subject: vt: selection, push sel_lock up

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

Quoted 11 lines

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.

Quoted 8 lines

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

Quoted 6 lines

-> #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

Quoted 8 lines

-> #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

Quoted 4 lines

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 <[email protected]>
Reported-by [email protected]
Fixes 07e6124a1a46 ("vt: selection, close sel_buffer race")
Cc stable <[email protected]>
Link https://lore.kernel.org/r/[email protected] open here
Signed-off-by Greg Kroah-Hartman <[email protected]>
---
 drivers/tty/vt/selection.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)
drivers/tty/vt/selection.c +7 -9
index 9126a01290ea..d7d2e4b844bc 100644
@@ -214,7 +214,6 @@ static int __set_selection_kernel(struct tiocl_selection *v, struct tty_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 tiocl_selection *v, struct tty_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 tiocl_selection *v, struct tty_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 tiocl_selection *v, struct tty_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 tiocl_selection *v, struct tty_struct *
 		}
 	}
 	sel_buffer_lth = bp - sel_buffer;
-unlock:
-	mutex_unlock(&sel_lock);
+
 	return ret;
 }
 

@@ -348,9 +344,11 @@ int set_selection_kernel(struct tiocl_selection *v, struct tty_struct *tty)
 {
 	int ret;
 
+	mutex_lock(&sel_lock);
 	console_lock();
 	ret = __set_selection_kernel(v, tty);
 	console_unlock();
+	mutex_unlock(&sel_lock);
 
 	return ret;
 }
Signature · 1 line
2.25.1