Re: [PATCH v2] usb: gadget: u_serial: pin ports while TTYs are installed

[Krystian Kaniewski <krystianmkaniewski@gmail.com>]

This patch also fixes another syzbot report with the same cause,
"KASAN: slab-use-after-free Write in release_tty":
https://syzkaller.appspot.com/bug?extid=bca09f5d8b843bbf7571

It is the close side of the same lifetime bug. gs_close() drops
port->port.count to zero, gserial_free_line() frees the gs_port, and
release_tty() then writes tty->port->itty in the freed memory. The
tty_init_dev() report is the same race on the open side. With this
patch the TTY holds a tty_port reference from gs_install() to
gs_cleanup(), so the port outlives both paths.

Tested in QEMU on the base of this patch (usb-next bdf12cde3083) with
the syzbot configuration and C reproducer of that report. Without the
patch KASAN reported the use-after-free in release_tty() within a
second in each of five boots. With the patch five runs of 300 seconds
each (about 8,600 reproducer iterations) showed no reports.

If the patch is accepted, these tags would close that report as well:

Reported-by: syzbot+bca09f5d8b843bbf7571@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=bca09f5d8b843bbf7571

Thanks,
--
Krystian Kaniewski