Re: [PATCH] tty: fix data race in n_tty_receive_buf_common

[Kohli, Gaurav]

Hi Alan,

Sorry correcting the typo here:
+retval =Â tty_ldisc_lock(tty, 5 * HZ);
+if (retval)
+ÂÂÂÂ ÂÂÂ goto err_release_lock;
tty->port->itty = tty;
/*
* Structures all installed ... call the ldisc open routines.
* If we fail here just call release_tty to clean up. No need
* to decrement the use counts, as release_tty doesn't care.
*/
retval = tty_ldisc_setup(tty, tty->link);
ÂÂÂ ÂÂÂ if (retval)
ÂÂÂ ÂÂÂ ÂÂÂ goto err_release_tty;
tty_ldisc_unlock(tty);
err_release_tty:
tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
ÂÂÂ retval, idx);
+err_release_lock;
+tty_unlock(tty);
+release_tty(tty, idx);
+tty_ldisc_unlock(tty);
+return ERR_PTR(retval);

On 1/6/2018 1:44 AM, Kohli, Gaurav wrote:
> Hi Alan,
>
>
> On 1/5/2018 7:45 PM, Alan Cox wrote:
> > > But in above case , there we can hit another race, if we have a 
> > > sequence
> > > like this
> > > tty_init_dev->alloc_tty_struct -> tty_ldisc_init -> this will 
> > > initialize
> > > ldisc ,
> > > but at this moment disc_data is still NULL
> > >
> > > And if flush_to_ldisc comes in between, it will take ldisc reference 
> > > and
> > > proceeds receive buffer.
> > So you need to move the lock up one line to protect the assignment to
> > tty->port->itty. We can do that.
> >
> > At that point your flush_to_ldisc should see either port->itty = NULL 
> > or a
> > valid initialized ldisc.
>
> Yes , with little modification this should work.
>
> +retval =Â tty_ldisc_lock(tty, 5 * HZ);
> +if (retval)
> +ÂÂÂÂ ÂÂÂ goto err_release_lock;
> tty->port->itty = tty;
> /*
> * Structures all installed ... call the ldisc open routines.
> * If we fail here just call release_tty to clean up. No need
> * to decrement the use counts, as release_tty doesn't care.
> */
> retval = tty_ldisc_setup(tty, tty->link);
> ÂÂÂ ÂÂÂ if (retval)
> ÂÂÂ ÂÂÂ ÂÂÂ goto err_release_tty;
> tty_ldisc_unlock(tty);
> err_release_tty:
> tty_unlock(tty);
> tty_info_ratelimited(tty, "ldisc open failed (%d), clearing slot %d\n",
> ÂÂÂ retval, idx);
> ÂÂÂ release_tty(tty, idx);
> ÂÂÂ return ERR_PTR(retval);
>
> +err_release_lock;
> +tty_ldisc_unlock(tty);
> +return ERR_PTR(retval);
>
> Please let me know if above modification seems fine , then we can 
> upload this and try reproduction of Bug.
>
> Regards
> Gaurav

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