linux-kernel - Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race condition

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Message-ID: <CADVatmMK=dCaAzdHbmQ6Qj5gqcvSiL3BRZDL6jnEM2G_C3pUpw@mail.gmail.com>
Date:   Tue, 17 Dec 2019 11:47:25 +0000
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race condition

Hi Greg,

On Thu, Dec 12, 2019 at 11:15 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Tue, Dec 10, 2019 at 11:41:47AM +0000, Sudip Mukherjee wrote:
> > Hi Jiri,
> >
> > On Fri, Nov 22, 2019 at 10:05:09AM +0100, Jiri Slaby wrote:
> > > On 21. 11. 19, 22:01, Sudip Mukherjee wrote:
> > > > Hi Greg,
> > > >
> > > > On Thu, Nov 21, 2019 at 05:41:38PM +0100, Greg Kroah-Hartman wrote:
> > > >> On Thu, Nov 21, 2019 at 03:22:39PM +0000, Sudip Mukherjee wrote:
> > > >>> There seems to be a race condition in tty drivers and I could see on
> > > >>> many boot cycles a NULL pointer dereference as tty_init_dev() tries to
> > > >>> do 'tty->port->itty = tty' even though tty->port is NULL.
> > <snip>
> > > >>>
> > > >>> uart_add_one_port() registers the console, as soon as it registers, the
> > > >>> userspace tries to use it and that leads to tty_open() but
> > > >>> uart_add_one_port() has not yet done tty_port_link_device() and so
> > > >>> tty->port is not yet configured when control reaches tty_init_dev().
> > > >>
> > > >> Shouldn't we do tty_port_link_device() before uart_add_one_port() to
> > > >> remove that race?  Once you register the console, yes, tty_open() can
> > > >> happen, so the driver had better be ready to go at that point in time.
> > > >>
> > > >
> > > > But tty_port_link_device() is done by uart_add_one_port() itself.
> > > > After registering the console uart_add_one_port() will call
> > > > tty_port_register_device_attr_serdev() and tty_port_link_device() is
> > > > called from this. Thats still tty core.
> > >
> > > Interferences of console vs tty code are ugly. Does it help to simply
> > > put tty_port_link_device to uart_add_one_port before uart_configure_port?
> >
> > sorry for the late response, got busy with an out-of-tree driver.
> >
> > It fixes the problem if I put tty_port_link_device() before
> > uart_configure_port(). Please check the attached patch and that
> > completely fixes the problem. Do you want me to send a proper patch for
> > it or do you want me to check more into it?
>
> This looks a lot more sane to me, can you resend it in proper format so
> that I can apply it?

https://lore.kernel.org/lkml/20191212131602.29504-1-sudipm.mukherjee@gmail.com/


-- 
Regards
Sudip