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    Subject[PATCH 3.16 223/245] vt: selection, push sel_lock up
    3.16.83-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    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")
    Link: https://lore.kernel.org/r/20200228115406.5735-2-jslaby@suse.cz
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    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
    @@ -205,7 +205,6 @@ static int __set_selection(const struct
    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
    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
    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
    if (!bp) {
    printk(KERN_WARNING "selection: kmalloc() failed\n");
    clear_selection();
    - ret = -ENOMEM;
    - goto unlock;
    + return -ENOMEM;
    }
    kfree(sel_buffer);
    sel_buffer = bp;
    @@ -329,8 +326,7 @@ static int __set_selection(const struct
    }
    }
    sel_buffer_lth = bp - sel_buffer;
    -unlock:
    - mutex_unlock(&sel_lock);
    +
    return ret;
    }

    @@ -338,9 +334,11 @@ int set_selection(const struct tiocl_sel
    {
    int ret;

    + mutex_lock(&sel_lock);
    console_lock();
    ret = __set_selection(v, tty);
    console_unlock();
    + mutex_unlock(&sel_lock);

    return ret;
    }
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