Re: [PATCH] n_tty: Remove LINEMODE support

[Howard Chu]

Peter Hurley wrote:
> Hi Howard,
>
> On 01/18/2015 05:09 PM, Howard Chu wrote:
> > Peter Hurley wrote:
> > > Commit 26df6d13406d1 ("tty: Add EXTPROC support for LINEMODE") added
> > > the undocumented EXTPROC input processing mode, which ignores the ICANON
> > > setting and forces pty slave input to be processed in non-canonical
> > > mode.
> > >
> > > Although intended to provide a transparent mechanism for local line
> > > edit with telnetd (and other remote shell protocols), the transparency
> > > is limited.
> > >
> > > Userspace usage is abandoned; telnetd does not even compile with
> > > LINEMODE support. readline/bash and sshd never supported this.
> >
> > I object to this. Code for all of the above exists and works. I use this code daily.
> >
> > https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=585527
> > http://lists.gnu.org/archive/html/bug-readline/2011-01/msg00004.html
> > https://github.com/hyc/OpenSSH-LINEMODE
> >
> > The lack of LINEMODE support in upstream sshd can only be considered a security hole.
> >
> > http://www.metzdowd.com/pipermail/cryptography/2015-January/024288.html
>
> These are all bug reports about userspace _not_ supporting this extension.

Bug reports *with working patches* attached. And the fact remains that 
not supporting this feature *is* a security liability.

> Where is a working userspace consumer of this interface?

The OpenSSH fork on github is a full working client and server using 
this interface.

> I seriously doubt this works reliably.
> What happens when the pty slave reader is in canonical mode and gets unterminated
> input because only a portion of the input is available yet? The way this is
> coded does _not_ require line termination before returning data to userspace.

Userspace already has to deal with incomplete lines if the input line is 
longer than the input buffer.

> Also, ioctl(FIONREAD) doesn't match what read() returns, nor that poll()/select()
> indicated input was available.

Hm, I think you're mistaken about poll/select.

    if ((!ldata->icanon && (ldata->read_cnt >= tty->minimum_to_wake)) ||
        L_EXTPROC(tty)) {
        kill_fasync(&tty->fasync, SIGIO, POLL_IN);
        if (waitqueue_active(&tty->read_wait))
            wake_up_interruptible(&tty->read_wait);
    }


--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/
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