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

From: Andy Lutomirski
Date: Fri Nov 06 2015 - 12:15:43 EST

On Fri, Nov 6, 2015 at 7:53 AM, Theodore Ts'o <tytso@xxxxxxx> wrote:
> 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.

What could be achievable, at least on some distros and with some
userspace changes, is to get rid of all privilege elevation on exec.
That is, turn off SUID, use ambient capabilities instead of file
capability masks, and use a systemwide privilege broker.

Of course, in this setting, the current sudo tool wouldn't work. But
there's nothing fundamental stopping someone from writing a mostly
drop-in sudo replacement that authenticates itself to a daemon and
asks the daemon to fork a privileged child on the sudo-replacement's

It would probably be a lot safer in the long run, because all of the
environment (env vars and everything else about execution state)
sanitation issues that plague setuid programs become non-issues.

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