linux-kernel - Re: [PATCH] Add supplementary UIDs, and getusers/setusers system calls

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Message-ID: <CALCETrXxqcJd9=9c+HNMYmO5V+pV+EpjTA0NVvC6peGv_CyWuQ@mail.gmail.com>
Date:	Thu, 20 Nov 2014 10:18:08 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	linux-man <linux-man@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Josh Triplett <josh@...htriplett.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Add supplementary UIDs, and getusers/setusers system calls

On Nov 20, 2014 7:16 AM, "Eric W. Biederman" <ebiederm@...ssion.com> wrote:
>
> Josh Triplett <josh@...htriplett.org> writes:
>
> > Analogous to the supplementary GID list, the supplementary UID list
> > provides a set of additional user credentials that a process can act as.
> > A process with CAP_SETUID can set its UID list arbitrarily; a process
> > without CAP_SETUID can only reduce its UID list.
> >
> > This allows each user to have a set of UIDs that they can then use to
> > further sandbox individual child processes without first escalating to
> > root to change UIDs.  For instance, a PAM module could give each user a
> > block of UIDs to work with.
>
> A couple of quick comments on this patch.
>
> 1) user namespaces already allow you to do this.

I thought you could only map your fsuid. Can you set fsuid to a
supplementary group?

>
> 2) After having looked at the group case I am afraid this intersects in
>    an unfortunate way with user namespaces.
>
> 3) This intersects in a very unfortunate way with setresuid.
>    Applications that today know they are dropping all privileges
>    won't be dropping all privielges with this change.  Which sounds like
>    a recipe for a security exploit to me.
>
> Eric
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