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

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Date:   Tue, 28 Nov 2017 17:04:40 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Mahesh Bandewar
         (महेश बंडेवार) <maheshb@...gle.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        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

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...