Re: Security: information leaks in /proc enable keystroke recovery

[Theodore Tso]

On Sat, Aug 15, 2009 at 03:21:27PM -0700, David Wagner wrote:
> At Usenix Security 2009, two researchers announced last week a new
> security vulnerability in multi-user Linux systems.  They demonstrated
> that one user can, in many cases, recover partial information about
> the keystrokes that another user types into applications running on
> that system.  For instance, they demonstrate how a malicious user can
> recover partial information about SSH passwords typed by other users,
> reducing the password search space by a factor of 250-2000x in
> their experiments.  Thus, this could facilitate password recovery.
> 
> In a nutshell, they exploit the fact that many files in /proc are
> world-readable yet contain sensitive information that can leak information
> about inter-keystroke timings.  For instance, /proc/$PID/stat reveals the
> ESP and EIP registers of the associated process, and is world-readable.
> /proc/pid/status is also mentioned as revealing information that could
> be exploited in these attacks.

A configuration option which defaults to disabling ESP and EIP would
be a simple way to prevent this specific instance of information
leakage.  The problem is there are other files that might reveal
timing information, but which are very useful for a system
administrator.  A key example of this is /proc/$pid/wchan, which is
responsible for the WCHAN column is a ps listing.

Maybe the better example, othter than making some of these files
non-world-readable, or zeroing out certain fields in /proc/$PID/stat,
is to put in a limit to how quickly specific files can be queried.  If
a process's /proc files which could be used to extract keystroke
timing information, is accessed more than say, ten times a second, a
random delay could be added before the information is returned.

Essentially, we can treat this as a covert channel that needs to be
restricted off, without necessarily completely removing the ability
for a system administrator to see what processes are blocked on via
"ps lx".

							- Ted
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