linux-kernel - Re: [PATCH v4] uart: fix race between uart_put_char() and uart

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Message-ID: <20180712141314.GB13192@cisco.cisco.com>
Date:   Thu, 12 Jul 2018 08:13:14 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        "open list:SERIAL DRIVERS" <linux-serial@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Serge E . Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v4] uart: fix race between uart_put_char() and
 uart_shutdown()

Hi Andy,

On Thu, Jul 12, 2018 at 12:03:08PM +0300, Andy Shevchenko wrote:
> On Wed, Jul 11, 2018 at 7:07 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> 
> > Anyway, since the lock is not acquired, if uart_shutdown() is called, the
> > last chunk of that function may release state->xmit.buf before its assigned
> > to null, and cause the race above.
> >
> > To fix it, let's lock uport->lock when allocating/deallocating
> > state->xmit.buf in addition to the per-port mutex.
> 
> >     * use the result of uart_port_lock() in uart_shutdown() to avoid
> >       uninitialized warning
> >     * don't use the uart_port_lock/unlock macros in uart_port_startup,
> >       instead test against uport directly; the compiler can't seem to "see"
> >       through the macros/ref/unref calls to not warn about uninitialized
> >       flags. We don't need to do a ref here since we hold the per-port
> >       mutex anyway.
> 
> > +       if (uport)
> > +               spin_lock_irqsave(&uport->lock, flags);
> 
> > +       if (uport)
> > +               spin_unlock_irqrestore(&uport->lock, flags);
> 
> At some point it It was uart_port_lock()/uart_port_unlock(), and you
> changed to simple spin lock. The macro also take reference to the
> port. Do we aware about that here?

I don't think so, the commit message you quoted above says,

> We don't need to do a ref here since we hold the per-port mutex
> anyway.

Cheers,

Tycho