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

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Date:	Thu, 20 Nov 2014 09:14:50 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Ts'o <tytso@....edu>,
	Kees Cook <keescook@...omium.org>, mtk.manpages@...il.com,
	linux-api@...r.kernel.org, linux-man@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Add supplementary UIDs, and getusers/setusers system calls

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.

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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