mergemaster and better support for ezjails

[Warren Block]

On Sun, 13 Jul 2014, Mateusz Guzik wrote:

> On Sat, Jul 12, 2014 at 08:08:52PM -0600, Warren Block wrote:
>> A couple of patches to make mergemaster work better with ezjails.
>>
>> These are only very superficially tested.  Feedback welcome.
>>
>> 1. If /etc/mergemaster.rc exists in the jail, it is sourced.  This
>>    allows IGNORE_FILES to be set in the jail.  And other settings, but
>>    that's the one I wanted.
>>
>
> How exactly does it work?
>
> Is jailed root allowed to create /etc/mergemaster.rc?

Yes.  Or at least I don't know of anything preventing that.

> If so, that would be a jail escape vector - an attacker puts commands they
> want to execute inside and mergemaster sourcing the file will trigger
> executing them.

Ouch.  Seems obvious now that you mention it.  Probably mergemaster.rc 
should have a defined format rather than being sourced anyway.

Another way to implement ignored files would be to extend the 
definitions in (the host's) /etc/mergemaster.rc to include ignored files 
by jail name or full path.

Full paths do not work presently because IGNORE_FILES just deletes the 
temporary file so it is not compared.

> In fact running mergemaster from "outside" on an untrusted jail seems
> like a security weakness even without jailed-root controlled rc file
> since they can try to do something fishy with symlinks which now resolve
> to stuff on the host.
>
> The following should be safe enough:
> - have a dedicated RO jail
> - mount to-be-updated jail under /mnt/jail or whatever
> - mount sources/whatever RO under /usr/src or whatever
> - run update process from inside dedicated RO jail

Thank you!