Re: proc hidepid=2 and SGID programs

[Vasiliy Kulikov <segoon@openwall.com>]

On Sun, Sep 15, 2013 at 01:58 -0700, Christian Kujau wrote:
> Vasiliy Kulikov <segoon@openwall.com> wrote:
> >> But still, I wonder if this is 
> >> intended behaviour.
> >
> >Yes.
> >
> >If you think such side channel attacks are something you don't care,
> >just turn hidepid off.  That's why it is an option.
> >
> >If you want to turn it off for some users, use gid=XXX.
> 
> Maybe my initial question got lost in the noise: I merely wondered why "pgrep sgid-program" returned nothing but "kill pics off stiff program" was possible. Sure, if that's intended behavior, so be it. I just don't understand the (technical) reasoning behind this.

If process A may ptrace process B, A may kill B.  In this case A may see
any information about B.

If process A may not ptrace process B, A probably still may kill B.  But
A may not see any information about B.

In sense of information gathering hidepid doesn't differ setgid'ed
processes and common processes of another user.  As *some* privileges
differ between a subject and an object, they are considered as being in
different security domains.  Information leakage crossing the
interdomain border between these domains might help an attacker, so it
is denied.

-- 
Vasily Kulikov
http://www.openwall.com - bringing security into open computing environments