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From:   Wander Costa <wcosta@redhat.com>
Date:   Thu, 17 Mar 2022 09:22:39 -0300
Message-ID: <CAAq0SUmuwc7KZWQOwoQfDrX+os8TVRhdr2pcXiwFNAgGCeRi=g@mail.gmail.com>
Subject: Re: [PATCH v4 5/5] serial/8250: Only use fifo after the port is
 initialized in console_write
To:     Jiri Slaby <jirislaby@kernel.org>
Cc:     Wander Lairson Costa <wander@redhat.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Johan Hovold <johan@kernel.org>,
        "Maciej W. Rozycki" <macro@orcam.me.uk>,
        Serge Semin <fancer.lancer@gmail.com>,
        Lukas Wunner <lukas@wunner.de>,
        =?UTF-8?Q?Pali_Roh=C3=A1r?= <pali@kernel.org>,
        "open list:SERIAL DRIVERS" <linux-serial@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        =?UTF-8?Q?Andr=C3=A9_Goddard_Rosa?= <andre.goddard@gmail.com>,
        Sudip Mukherjee <sudipm.mukherjee@gmail.com>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Jon Hunter <jonathanh@nvidia.com>, phil@raspberrypi.com
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On Thu, Mar 17, 2022 at 4:06 AM Jiri Slaby <jirislaby@kernel.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@redhat.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
>