From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Triplett Subject: Re: [PATCH 2/2] groups: Allow unprivileged processes to use setgroups to drop groups Date: Sat, 15 Nov 2014 11:29:25 -0800 Message-ID: <20141115192924.GB19060@thin> References: <3ccec8a13019b5e8ce7b1d7889677b778b070dc8.1416041823.git.josh@joshtriplett.org> <0895c1f268bc0b01cc6c8ed4607d7c3953f49728.1416041823.git.josh@joshtriplett.org> <87d28osceg.fsf@x220.int.ebiederm.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <87d28osceg.fsf-JOvCrm2gF+uungPnsOpG7nhyD016LWXt@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Eric W. Biederman" Cc: Andrew Morton , Kees Cook , mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org, linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-man-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: linux-api@vger.kernel.org On Sat, Nov 15, 2014 at 09:37:27AM -0600, Eric W. Biederman wrote: > Josh Triplett writes: > > > Currently, unprivileged processes (without CAP_SETGID) cannot call > > setgroups at all. In particular, processes with a set of supplementary > > groups cannot further drop permissions without obtaining elevated > > permissions first. > > > > Allow unprivileged processes to call setgroups with a subset of their > > current groups; only require CAP_SETGID to add a group the process does > > not currently have. > > A couple of questions. > - Is there precedence in other unix flavors for this? I found a few references to now-nonexistent pages at MIT about a system with this property, but other than that no. I've also found more than a few references to people wanting this functionality. > - What motiviates this change? I have a series of patches planned to add more ways to drop elevated privileges without requiring a transition through root to do so. That would improve the ability for unprivileged users to run programs sandboxed with even *less* privileges. (Among other things, that would also allow programs running with no_new_privs to further *reduce* their privileges, which they can't currently do in this case.) > - Have you looked to see if anything might for bug compatibilty > require applications not to be able to drop supplementary groups? I haven't found any such case; that doesn't mean such a case does not exist. Feedback welcome. The only case I can think of (and I don't know of any examples of such a system): some kind of quota system that limits the members of a group to a certain amount of storage, but places no such limit on non-members. However, the idea of *holding* a credential (a supplementary group ID) giving *less* privileges, and *dropping* a credential giving *more* privileges, would completely invert normal security models. (The sane way to design such a system would be to have a privileged group for "users who can exceed the quota".) If it turns out that a real case exists that people care about, I could easily make this configurable, either at compile time or via a sysctl. - Josh Triplett