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Subject[PATCH 5.4 087/168] vt: selection, push sel_lock up
Date
From: Jiri Slaby <jslaby@suse.cz>

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

> -> #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: 07e6124a1a46 ("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>

---
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;
}

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