linux-kernel - Re: [PATCHv2 2/2] userns: control capabilities of some user namespaces

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Message-ID: <CAF2d9jhCDasnf2tbaGA04MV5ygZ9o1FaVRCEU2TkHjZR7d1ifw@mail.gmail.com>
Date:   Tue, 28 Nov 2017 15:50:48 -0800
From:   Mahesh Bandewar (महेश बंडेवार) 
        <maheshb@...gle.com>
To:     "Serge E. Hallyn" <serge@...lyn.com>
Cc:     Mahesh Bandewar <mahesh@...dewar.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Kernel-hardening <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Eric Dumazet <edumazet@...gle.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCHv2 2/2] userns: control capabilities of some user namespaces

On Tue, Nov 28, 2017 at 3:04 PM, Serge E. Hallyn <serge@...lyn.com> wrote:
> Quoting Mahesh Bandewar (महेश बंडेवार) (maheshb@...gle.com):
> ...
>> >> diff --git a/security/commoncap.c b/security/commoncap.c
>> >> index fc46f5b85251..89103f16ac37 100644
>> >> --- a/security/commoncap.c
>> >> +++ b/security/commoncap.c
>> >> @@ -73,6 +73,14 @@ int cap_capable(const struct cred *cred, struct user_namespace *targ_ns,
>> >>  {
>> >>       struct user_namespace *ns = targ_ns;
>> >>
>> >> +     /* If the capability is controlled and user-ns that process
>> >> +      * belongs-to is 'controlled' then return EPERM and no need
>> >> +      * to check the user-ns hierarchy.
>> >> +      */
>> >> +     if (is_user_ns_controlled(cred->user_ns) &&
>> >> +         is_capability_controlled(cap))
>> >> +             return -EPERM;
>> >
>> > I'd be curious to see the performance impact on this on a regular
>> > workload (kernel build?) in a controlled ns.
>> >
>> Should it affect? If at all, it should be +ve since, the recursive
>> user-ns hierarchy lookup is avoided with the above check if the
>> capability is controlled.
>
> Yes but I expect that to be the rare case for normal lxc installs
> (which are of course what I am interested in)
>
>>  The additional cost otherwise is this check
>> per cap_capable() call.
>
> And pipeline refetching?
>
> Capability calls also shouldn't be all that frequent, but still I'm
> left wondering...

Correct, and capability checks are part of the control-path and not
the data-path so shouldn't matter but I guess it doesn't hurt to
find-out the number. Do you have any workload in mind, that we can use
for this test/benchmark?