[PATCH 1/3] ima: keep the integrity state of open files up to date

[Eric Biggers]

On Mon, Sep 16, 2019 at 02:45:56PM +0300, Janne Karhunen wrote:
> On Sun, Sep 15, 2019 at 11:24 PM Eric Biggers <ebiggers at kernel.org> wrote:
> 
> > > > This still doesn't make it crash-safe.  So why is it okay?
> > >
> > > If Android is the load, this makes it crash safe 99% of the time and
> > > that is considerably better than 0% of the time.
> > >
> >
> > Who will use it if it isn't 100% safe?
> 
> I suppose anyone using mutable data with IMA appraise should, unless
> they have a redundant power supply and a kernel that never crashes. In
> a way this is like asking if the ima-appraise should be there for
> mutable data at all. All this is doing is that it improves the crash
> recovery reliability without taking anything away.

Okay, so why would anyone use mutable data with IMA appraise if it corrupts your
files by design, both with and without this patchset?

> 
> Anyway, I think I'm getting along with my understanding of the page
> writeback slowly and the journal support will eventually be there at
> least as an add-on patch for those that want to use it and really need
> the last 0.n% reliability. Note that even without that patch you can
> build ima-appraise based systems that are 99.999% reliable just by

On what storage devices, workloads, and filesystems is this number for?

> having the patch we're discussing here. Without it you would be orders
> of magnitude worse off. All we are doing is that we give it a fairly
> good chance to recover instead of giving up without even trying.
> 
> That said, I'm not sure the 100% crash recovery is ever guaranteed in
> any Linux system. We just have to do what we can, no?
> 

Filesystems implement consistency mechanisms, e.g. journalling or copy-on-write,
to recover from crashes by design.  This patchset doesn't implement or use any
such mechanism, so it's not crash-safe.  It's not clear that it's even a step in
the right direction, as no patches have been proposed for a correct solution so
we can see what it actually involves.

- Eric