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

[ebiederm@xmission.com (Eric W. Biederman)]

Josh Triplett <josh@joshtriplett.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