Peter Hurley wrote:

Hi Howard,

On 01/18/2015 05:09 PM, Howard Chu wrote:

quoted

Peter Hurley wrote:

quoted

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/