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

From: Eric W. Biederman
Date: Thu Nov 20 2014 - 10:16:18 EST

Josh Triplett <josh@xxxxxxxxxxxxxxxx> 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.

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