Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: [KERNEL] Re: Kernel 4.3 breaks security in systems using capabilities

From: Theodore Ts'o
Date: Fri Nov 06 2015 - 10:53:23 EST

On Fri, Nov 06, 2015 at 02:58:36PM +0100, Klaus Ethgen wrote:
> But that left out completely the, I think more important, usecase of
> _removing_ SUID completely and _replacing_ it with very tight capability
> setting. And that is what I always talked about.

I don't believe this is ever going to be possible. And I'm not
talking about it from a technical perspective, but from a practical
and cultural perspective.

The problem with removing SUID and inheritance completely is that you
have to anticipate all possible use cases where a system administrator
might want to use a root shell. This means analyzing all possible use
cases for all possible system administrators how they might need to
use a root shell to fix or management a system, and then either (a)
provide a new, specialized tool that solves that particular use case,
while respecting the rules of least privilege, or (b) figure out how
to expand that executable's fI mask, and worse, if that executable
fork and exec's helper programs, those helper programs will need to
have expanded fI masks. And that's if all of the commands that a
system administrator needs to run are compiled executables. Now
consider what happens when a system administrator needs to run python,
perl, or shell scripts with elevated privileges.

You maybe can solve this in a very restricted environment; say, one
really dedicated user who can tweak his or her own's fI masks because
hopefully he or she can anticipate all possible use cases. But in the
general case? For a general purpose distribution? Good luck with that.

Schemes like this could work if you are willing to essentially outlaw
all administrative shell/perl/python scripts. Otherwise, the fact
that /bin/sh, /bin/perl, and /bin/python essentially have an unlimited
fI mask means the whole system has a hole you can drive a truck hole,
in which case, what's the point of going through the whole effort?

In the light of that, using things like ambient capabilities, or using
setuid binary that immediately drops all caps that it needs, is
probably the best we're going to get.


- Ted
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