Re: Fwd: kernel: multiple flaws allowing to sniff keystrokes timings

[Greg KH]

On Wed, Nov 09, 2011 at 09:09:46PM +0400, Vasiliy Kulikov wrote:
> Hi,
> 
> On Wed, Nov 09, 2011 at 08:11 -0800, Greg KH wrote:
> > On Wed, Nov 09, 2011 at 01:05:54PM +0800, Eugene Teo wrote:
> > > 1) https://lkml.org/lkml/2011/11/7/340
> > > 
> > > "/proc/interrupts contains the number of emitted interrupts, which
> > > should not be world readable.  The information about keyboard interrupts
> > > number may be used to learn the precise number of characters
> > > in users' passwords by simply watching the changes of number of emitted
> > > interrupts during the life of gksu-like programs."
> > > 
> > > PoC: http://www.openwall.com/lists/oss-security/2011/11/07/9
> > > 
> > > Vulnerable: all Linux versions, all distros with procfs mounted.
> > > 
> > > (The patch misses the same infoleak via /proc/stat, which must be closed
> > > too.)
> > 
> > The patch is also buggy, and might be reverted at any moment :(
> 
> Hm?  This patch was never applied (at least I haven't received a tip for it).
> 
> You're probably talking about /proc/$PID/fd* with a weird deplock
> warning, which is irrelevant.

Oops, sorry, you are correct.

> > > 2) https://lkml.org/lkml/2011/11/7/355
> [...]
> > But we really can't change this as it would break compatibility, as Alan
> > Cox pointed out, so what can be done here?
> > 
> > > 3) https://lkml.org/lkml/2011/11/8/136
> [...]
> > revoke() is still a dream (I looked into it and no other OS implements
> > it as we have talked about it so I still fail to see how it would really
> > work), so don't hold your breath waiting for that to happen...
> 
> Sad.  I see only two solutions then:
> 
> 1) show zero fields to unprivileged users (for /proc/interrupts and
> /proc/stat it is CAP_SYS_ADMIN, for /proc/$PID/{stat,sched,schedstat} it
> is ptrace_may_access(), for ttys it is uid check) and real values for
> privileged.  The same technique is used in /proc/$PID/sched for eip/esp
> values.

That makes sense to a point, users will wonder why they aren't seeing
interrupts anymore, which is a valuable debug tool.

All that is leaking here is the number of keystrokes at most, right?

> 2) completely deny the syscalls in questions for unprivileged users.
> Not a way to go with /proc/$PID/* as it would break top and other cpu
> monitors.

Yes, not good.

> The solution for /proc/$PID/* could come from the applications themselves,
> i.e. they could create additional CPU noise while processing sensible
> information like passwords (like some crypto software does to protect
> against timing attacks), but it is crazy to expect this from all developers.
> Probably it would not even fix the issue as the activity can be still
> learned from other programs they interact with via the same /proc/$PID/*
> files or other implicit information sources.

If we provide a "good" library function for keyboard password usage, we
could get other applications to use it, but yes, it is a tough issue :(

greg k-h
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