Re: [PATCH 3/3] ima: use fs method to read integrity data (updated patch description)

From: Mimi Zohar
Date: Sun Sep 17 2017 - 12:16:12 EST

On Sun, 2017-09-17 at 08:28 -0700, Linus Torvalds wrote:
> On Sun, Sep 17, 2017 at 8:17 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> >
> > Only for direct I/O, and IMA and direct I/O don't work together.
> > From ima_collect_measurement:
> >
> > if (file->f_flags & O_DIRECT) {
> > audit_cause = "failed(directio)";
> > result = -EACCES;
> > goto out;
> > }
> That's not the issue.
> The issue is that somebody else can come in - using direct IO - at the
> same time as the first person is collecting measurements, and thus
> race with the collector.
> So now the measurements are not trustworthy any more.

Unless I'm missing something, that would only be possible with an IMA
policy rule that permits direct IO (eg. permit_directio). ÂOtherwise
the direct IO is denied.

> > Well, that's exactly the point of the new ->integrity_read routine
> > I proposed and prototype. The important thing is that it is called
> > with i_rwsem held because code mugh higher in the chain already
> > acquired it, but except for that it's entirely up to the file system.
> .. and *my* point is that it's the wrong lock for actually checking
> integrity (it doesn't actually guarantee exclusion, even though in
> practice it's almost always the case), and so we're adding a nasty
> callback that in 99% of all cases is the same as the normal read, and
> we *could* have just added it with a RWF flag instead.
> Is there some reason why integrity has to use that particular lock
> that is so inconvenient for the filesystems it wants to check?

Originally IMA had its own lock (iint->mutex), prior to IMA-appraisal
being upstreamed. ÂWith a separate lock, the iint->mutex and i_rwsem
would be taken in reverse order in process_measurements() and in the
setxattr, chown, chmod syscalls.

I'm at the airport on my way back home.