[systemd-devel] systemd shutdown vs ostree

[Zbigniew Jędrzejewski-Szmek]

On Fri, Sep 13, 2013 at 08:50:06AM +0100, Colin Guthrie wrote:
> 'Twas brillig, and Lennart Poettering at 13/09/13 00:02 did gyre and gimble:
> > On Thu, 12.09.13 23:31, Colin Guthrie (gmane at colin.guthr.ie) wrote:
> > 
> >>
> >> 'Twas brillig, and Lennart Poettering at 12/09/13 21:51 did gyre and gimble:
> >>> On Thu, 12.09.13 21:27, Colin Guthrie (gmane at colin.guthr.ie) wrote:
> >>>
> >>>>> So, as mentioned in the other thread, /usr should probably be on some
> >>>>> "OS resource blacklist" or so, and not attempted to be shutdown.
> >>>>>
> >>>>> But unmounting /var during shutdown should actually work. The only thing
> >>>>> that currently stops this from working cleanly is that journald is
> >>>>> configured to stay around until the last moment, but currently has not
> >>>>> interface to tell it to move its logging back to /run from /var. When we
> >>>>> have that then the /var issue should go away for you, no?
> >>>>
> >>>> It would be quite nice to have an easy way for a compliant initrd to say
> >>>> it can and will handle /var unmounting (perhaps by dropping a
> >>>> var.mount.d/ snippet in the /run/systemd/system/ folder at boot?). 
> >>>
> >>> Yeah, such a drop-in should just work. Just add "DefaultDependencies=no"
> >>> in there, and maybe instead "Before=local-fs.target", and you should be set.
> >>
> >> Nice, I'm sure dracut could implement that then.
> >>
> >>>> This will allow us to get persistent logs in which the initrd can
> >>>> report it's progress until the last possible moment. This might help
> >>>> debugging any problems in this particular link of the shutdown chain.
> >>>
> >>> I think the really late shutdown logs should more liekly end up in some
> >>> EFI var and then flushed out on next boot or so...
> >>
> >> Nice plan. How would be able to "trust" that data tho'. Would it have to
> >> be signed in some way? Or could it be marked somehow as unreliable data?
> >> I'm not sure it really *matters* that much from a trust perspective as
> >> it's use would likely be 99% debugging, but all the same it would be
> >> nice if it were trustable.
> > 
> > Trust?  I mean, if you managed to upload data into EFI vars you are so
> > privileged you can fuck with us anyway, so not sure what you mean....
> 
> Well I was originally thinking about other OS's etc. and knowing that
> logs came from the right install, but then posed the question in a more
> generic sense, in part due to the cryptographic guarantees given by FSS.
> I just figured this was an interesting way in which someone could inject
> false data at a point where you have zero way to know "what went on"
> between the last shutdown and the current boot.
> 
> If is to be inherently trusted based on machine id then fine - just
> seems odd to put all the effort into FSS and then blindly inject data
> (possibly even core dumps which will be analysed later) and not mark it
> as potentially untrustworthy in some way.
FSS doesn't protect against fake data, it only allows you to detect when
data has been removed. So if I break into a machine with f-s-sealed logs,
I am still able to log anything I want, but the messages about my break-in
will be there, unless I remove some messages, in which case it will be visible
that there are no messages since some point. The same guarantee holds for
messages from pstore: you need root rights to write there. Even if there's
a second OS on the machine, if you have root right in that *other* OS, you
can write whatever you want to the logs in *this* OS, either through pstore
or directly by mounting /var. So pstore doesn't really change anything here.

Zbyszek