Crusty Blaa

After a couple of weeks work Caolan
and I have gotten together a prototype for a VNC based terminal services system which
also supports hotdesking. More details

here.

This is all part of a bigger plan to use VNC for the four different "remote desktop"
uses cases I think we have in GNOME:

• Terminals (a.k.a. thin clients) which display your desktop running on a
  terminal server
• Remote administration - allowing sysadmins to remotely help a user having
  difficulties with their desktop
• Remote access - you're at home and you need to do something on your machine
  in work like save a document you'd forgotten to save or something.
• Collaboration - a colleague in another office is working on a CAD drawing and
  he wants to demonstrate some of his ideas while talking to you on the phone
  about it

I'll post more of the technical details of how this should all work. Right now,
though, I need to go for a few pints and get away from this stuff :-)

• Up early; chewed mail;
• Amazing lack of listening on the xdg list from Michael along the lines
  of: "let me fix my problems right now with the least amount of pain to
  me. I don't mind if its a big hack which everyone else will copy". The
  encouraging thing from this is I suppose that in the past I would have
  taken this (IMHO fairly nonsensical/incomprehensible stalling position)
  as cast-iron proof of Michael just not listening; however, one can only
  conclude from this discussion that there is some sort of unhelpful,
  underlying ignoring the
  point
  going on ... nice to have this insight.

:-P

Okay, Havoc needs his pot leaf
head re-instanted. That
film has to be the most atrocious piece of work I've seen in a
long time ... "So bad it was good"? Yeah, right.

Someone else has been smoking some goodness:

Of course, Ulrich did know. The
System V ABI specification's ELF section contains the requirement
that \$LD_LIBRARY_PATH have those semantics

   a string that holds a list of directories, separated by colons
   (:). For example, the string /home/dir/lib:/home/dir2/lib: tells the
   dynamic linker to search first the directory /home/dir/lib, then
   /home/dir2/lib, and then the current directory to find dependencies.

  export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/some/other/dir

Do this instead

  if [ -n $LD_LIBRARY_PATH ]; then
    export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/some/other/dir
  else
    export LD_LIBRARY_PATH=/some/other/dir
  fi

Or even

  export LD_LIBRARY_PATH=${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}/some/other/dir

Kjartan pointed out a report of a gnome-session security vulnerability
to me earlier today. Now, it turns out to actually only be a
vulnerability in a script supplied with certain distributors' packages,
but that's not the interesting part.

The script contained something like this:

  export LD_LIBRARY_PATH=/opt/gnome/lib:$LD_LIBRARY_PATH

and the "vulnerability" was apparently that if \$LD_LIBRARY_PATH
was originally unset, then you get
LD_LIBRARY_PATH="/opt/gnome/lib:".

I couldn't for the life of me understand why it was actually a
vulnerability in the first place. It gnawed at me all day. After a bit
of poking at the runtime linker code in glibc though I realized that
setting \$LD_LIBRARY_PATH like that is effectively the same as
setting it to LD_LIBRARY_PATH="/opt/gnome/lib:./". The problem is
clear then - an attacker can use it to force a rogue, untrusted
library to be loaded that could do nasty things.

So, this whole LD_LIBRARY_PATH=":" causing "./" to be added to
the runtime linker's search path thing came as a big suprise to me and
to some others. I'd never heard of before, but it seems known to at
least some people and is "expected behavior". What worries me here is
that its likely most attackers know to look out for this and most
developers don't.

My long search for a normative reference for this behaviour failed. If
anyone knows where it comes from, I'm dead curious. I expect Ulrich
knows, though :-)

Another interesting thing to play with:

$> LD_DEBUG=help /lib/ld-linux.so.2
Valid options for the LD_DEBUG environment variable are:

  libs        display library search paths
  reloc       display relocation processing
  files       display progress for input file
  symbols     display symbol table processing
  bindings    display information about symbol binding
  versions    display version dependencies
  all         all previous options combined
  statistics  display relocation statistics
  help        display this help message and exit

I read a paper
from Ulrich some time ago that gave lots of details on how to write
shared libraries such that the runtime linker can more efficiently
resolve symbols etc. LD_DEBUG=statistics, among other things,
is useful there. I'd love to see someone with the time and interest
looking into this and analyzing GNOME libraries to see where we can
make improvements.