linux-kernel - Re: Kernel 4.3 breaks security in systems using capabilities

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Date:	Mon, 2 Nov 2015 19:38:40 +0100
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Klaus Ethgen <Klaus+lkml@...gen.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Andy Lutomirski <luto@...nel.org>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Kernel 4.3 breaks security in systems using capabilities

CC'ing patch authors.

On Mon, Nov 2, 2015 at 7:06 PM, Klaus Ethgen <Klaus+lkml@...gen.de> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA512
>
> Hi,
>
> I read recently about patch 58319057b7847667f0c9585b9de0e8932b0fdb08
> which made it into kernel 4.3 recently. And I have to say that I was
> shocked on how could such a patch that breaks normal use of capabilities
> make it into the kernel.
>
> Usually I have set very own crafted capabilities set to files instead of
> having them SUID root. With that, I have a comparable set of inheritable
> capabilities set for limited users. That allows me to nearly drop all
> SUID binaries and replace it by only giving the processes the
> capabilities they need but only if the users are allowed to act with
> that capabilities. Especially, and that is important, it inhibit any
> leak of rights to any forked process, be it indented or by a security
> problem of the binary.
>
> With the patch above, any process that is spawned by such a program will
> inherit the raised capabilities if it has no own filecapabilities set.
> Even worse, even every user made tool can be target for such
> escalations! That drives the benefits in security of capabilities over
> SUID ad-absurdum.
>
> Let me add here, that I disagree with Andy Lutomirski about the
> usefulness of capability inheritance in kernels before that patch. They
> was fully usefull to only allow selective capabilities if both, the
> binary and the user was allowed to use it. I never want to have any
> capabilities for processes that I did not allow them to have. Even
> worse, I never want any capabilities allowed for any shell. It is
> horrible to even think about such a possibility!.
>
>> Users with nonzero pA are unlikely to unintentionally leak that
>> capability. If they run programs that try to drop privileges, dropping
>> privileges will still work.
>
> Even that is naiv. There are only few programs out there that do
> actively drop privileges. Most are agnostic about capabilities. But this
> crappy patch introduce a need for _every_ tool to drop all capabilities
> right after start to stay in a secure system.
>
> So please revert that patch as fast as possible before it does some harm
> by getting into some real world systems!
>
> Regards
>    Klaus Ethgen
> - --
> Klaus Ethgen                              http://www.ethgen.ch/
> pub  4096R/4E20AF1C 2011-05-16   Klaus Ethgen <Klaus@...gen.de>
> Fingerprint: 85D4 CA42 952C 949B 1753  62B3 79D0 B06F 4E20 AF1C
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-- 
Thanks,
//richard
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