linux-kernel - Re: [PATCH v4 5/5] serial/8250: Only use fifo after the port is initialized in console

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Date:   Thu, 17 Mar 2022 09:22:39 -0300
From:   Wander Costa <wcosta@...hat.com>
To:     Jiri Slaby <jirislaby@...nel.org>
Cc:     Wander Lairson Costa <wander@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Johan Hovold <johan@...nel.org>,
        "Maciej W. Rozycki" <macro@...am.me.uk>,
        Serge Semin <fancer.lancer@...il.com>,
        Lukas Wunner <lukas@...ner.de>,
        Pali Rohár <pali@...nel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        André Goddard Rosa <andre.goddard@...il.com>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Jon Hunter <jonathanh@...dia.com>, phil@...pberrypi.com
Subject: Re: [PATCH v4 5/5] serial/8250: Only use fifo after the port is
 initialized in console_write

On Thu, Mar 17, 2022 at 4:06 AM Jiri Slaby <jirislaby@...nel.org> wrote:
>
> On 16. 03. 22, 15:36, Wander Lairson Costa wrote:
> > The serial driver set the value of uart_8250_port.fcr in the function
> > serial8250_config_port, but only writes the value to the controller
> > register later in the initalization code.
> >
> > That opens a small window in which is not safe to use the fifo for
> > console write.
> >
> > Make sure the port is initialized correctly before reading the FCR
> > cached value.
> >
> > Unfortunately, I lost track of who originally reported the issue. If
> > s/he is reading this, please speak up so I can give you the due credit.
> >
> > Signed-off-by: Wander Lairson Costa <wander@...hat.com>
> > ---
> >   drivers/tty/serial/8250/8250_port.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> > index 4acf620be241..7e2227161555 100644
> > --- a/drivers/tty/serial/8250/8250_port.c
> > +++ b/drivers/tty/serial/8250/8250_port.c
> > @@ -3416,6 +3416,7 @@ void serial8250_console_write(struct uart_8250_port *up, const char *s,
> >               !(up->capabilities & UART_CAP_MINI) &&
> >               up->tx_loadsz > 1 &&
> >               (up->fcr & UART_FCR_ENABLE_FIFO) &&
> > +             test_bit(TTY_PORT_INITIALIZED, &port->state->port.iflags) &&
>
> Cannot be port->state be NULL sometimes here?
>

IIUC, state is assigned at early port registration in
uart_add_one_port(), so this function wouldn't be called when state is
NULL. But I think it causes no harm to add an extra check. Thanks!

> >               /*
> >                * After we put a data in the FIFO, the controller will send
> >                * it regardless of the CTS state. Therefore, only use fifo
>
>
> --
> js
> suse labs
>