linux-kernel - Re: [PATCH] capabilities: Introduce CAP

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Date:   Sun, 24 May 2020 19:01:54 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Michał Cłapiński <mclapinski@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Dirk Petersen <dipeit@...il.com>,
        Christine Flood <chf@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        Aaron Goidel <acgoide@...ho.nsa.gov>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>,
        Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH] capabilities: Introduce CAP_RESTORE

On 5/22/2020 9:27 PM, Andrei Vagin wrote:
> On Fri, May 22, 2020 at 09:40:37AM -0700, Casey Schaufler wrote:
>> On 5/21/2020 10:53 PM, Adrian Reber wrote:
>>> There are probably a few more things guarded by CAP_SYS_ADMIN required
>>> to run checkpoint/restore as non-root,
>> If you need CAP_SYS_ADMIN anyway you're not gaining anything by
>> separating out CAP_RESTORE.
>>
>>>  but by applying this patch I can
>>> already checkpoint and restore processes as non-root. As there are
>>> already multiple workarounds I would prefer to do it correctly in the
>>> kernel to avoid that CRIU users are starting to invent more workarounds.
>> You've presented a couple of really inappropriate implementations
>> that would qualify as workarounds. But the other two are completely
>> appropriate within the system security policy. They don't "get around"
>> the problem, they use existing mechanisms as they are intended.
>>
> With CAP_CHECKPOINT_RESTORE, we will need to use the same mechanisms.

Then why call them out as objectionable "workarounds"?

> The problem is that CAP_SYS_ADMIN is too wide.

This is well understood, and irrelevant.

If we broke out CAP_SYS_ADMIN properly we'd have hundreds of
capabilities, and no one would be able to manage the capability
sets on anything. Just breaking out of CAP_SYS_ADMIN, especially
if the process is going to need other capabilities anyway, gains
you nothing.

>  If a process has
> CAP_SYS_ADMIN, it can do a lot of things and  the operation of forking a
> process with a specified pid isn't the most dangerous one in this case.
> Offten security policies don't allow to grant CAP_SYS_ADMIN to any
> third-party tools even in non-root user namespaces.
>