Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces

[Serge E. Hallyn]

> 3. Find a way to allow setgroups() in a user namespace while keeping
>    in mind the case of groups used for negative access control.
>    This was suggested by Josh Triplett and Geoffrey Thomas. Their idea was to
>    investigate adding a prctl() to allow setgroups() to be called in a user
>    namespace at the cost of restricting paths to the most restrictive
>    permission. So if something is 0707 it needs to be treated as if it's 0000
>    even though the caller is not in its owning group which is used for negative
>    access control (how these new semantics will interact with ACLs will also
>    need to be looked into).

I should probably think this through more, but for this problem, would it
not suffice to add a new prevgroups grouplist to the struct cred, maybe
struct group_info *locked_groups, and every time an unprivileged task creates
a new user namespace, add all its current groups to this list?