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

[Greg Kroah-Hartman]

On Tue, Dec 17, 2019 at 11:47:25AM +0000, Sudip Mukherjee wrote:
> Hi Greg,
> 
> On Thu, Dec 12, 2019 at 11:15 AM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx/

It's in my "to review" queue, don't worry, not lost :)