linux-kernel - Re: [PATCH] proc: use ns_capable instead of capable for timerslack

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Date:   Tue, 30 Oct 2018 10:40:27 -0600
From:   Benjamin Gordon <bmgordon@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, John Stultz <john.stultz@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Oren Laadan <orenl@...lrox.com>,
        Ruchi Kandoi <kandoiruchi@...gle.com>,
        Rom Lemarchand <romlem@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, Colin Cross <ccross@...roid.com>,
        Nick Kralevich <nnk@...gle.com>,
        Dmitry Shmidt <dimitrysh@...gle.com>,
        Elliott Hughes <enh@...gle.com>,
        Android Kernel Team <kernel-team@...roid.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] proc: use ns_capable instead of capable for timerslack_ns

On Thu, Oct 25, 2018 at 01:56:27PM -0500, Eric W. Biederman wrote:
> > Access to timerslack_ns is controlled by a process having CAP_SYS_NICE
> > in its effective capability set, but the current check looks in the root
> > namespace instead of the process' user namespace.  Since a process is
> > allowed to do other activities controlled by CAP_SYS_NICE inside a
> > namespace, it should also be able to adjust timerslack_ns.
> 
> The goal seems legitimate.  However the permission checks look wrong.
> 
> In particular the choice of user namespace should be
> "p->cred->user_ns".  This will limit this to tasks that have
> CAP_SYS_NICE in the same namespace as the task that is being modified.
> 
> Testing file->f_cred->user_ns it is testing whoever opened the file and
> that could be anyone.

Thanks, that seems like the right answer here.  I'll send a v2 to fix
it.

Thanks,
Benjamin