linux-kernel - Re: [RFC][PATCH] Fix cap_capable to only allow owners in the parent user namespace to have caps.

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Date:	Thu, 13 Dec 2012 18:36:02 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	containers@...ts.linux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-security-module@...r.kernel.org
Subject: Re: [RFC][PATCH] Fix cap_capable to only allow owners in the parent
 user namespace to have caps.

On Thu, Dec 13, 2012 at 6:33 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
>
> Andy thank you for your review.
>
> Andy Lutomirski <luto@...capital.net> writes:
>> This is confusing enough that I can't immediately tell whether it's
>> correct.  I think it's close but out of order.
>
> Yeah.  That is the trick. Figuring out how to write that code so it is
> correct and obvious.
>
> I have added a comment at the top and removed the extra variable I was
> adding.
>
> The order except for verifying a user_ns->parent is valid by checking
> for targ_ns == &init_user_ns doesn't make a difference.  Because
> cred->user_ns can only be one of targ_ns or targ_ns->parent.
>
>> Should this be transitive?
>
> Yes.
>>  I.e. suppose uid 1 owns a child of
>> init_user_ns and uid 2 (mapped in the first ns as the identity) owns
>> an inner ns.  Does uid 2 in the root ns have all caps inside?  I'd say
>> no, but I don't have a great argument for that.
>
> I also say no.  It is more code and it doesn't fit my nice small
> definition.  You have to be the owner and you have to be in the parent
> of the target user namespace.  Being able to remember the rules in
> your head is important.
>
>> But uid 1 presumably
>> does have caps because it could enter the parent with setns, then
>> change uid, then enter the child.
>
> Yes. uid 1 does have caps.
>
>> How about (severely whitespace damaged):
>
> You know that makes the termination condition a bit clearer.  But it
> looses the nice place to put a comment when we loop again.  This loop
> is just subtle enough that I want to preserve my comments.
>
> I think I must have put -EPERM towards the end for the same reason to
> make it clear that is the termination condition.
>
> In practice I think it is important to have the cap_raised case first,
> as that is the common case, and if we can be clear and still test that
> case first it means the code will be faster.  With my reordering it is
> obvious that nothing strange happens in the initial user namespace with
> the owner test after the exit when we are the initial user namespace.

Ah.  You are correct about the ordering.  I read it slightly wrong.

I'd still suggest using a variable like "here" instead of "targ_ns".
The latter is confusing because it changes on the second and later
iterations.

--Andy
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