Re: [PATCH 3/4] integrity: IMA as an integrity service provider

Dave Hansen
2008-11-21T17:39:15Z

On Thu, 2008-11-20 at 20:39 -0500, Mimi Zohar wrote:
> On Thu, 2008-11-20 at 13:22 -0800, Dave Hansen wrote:
> > On Thu, 2008-11-20 at 11:43 -0500, Mimi Zohar wrote:
> > >
> > > + /* Invalidate PCR, if a measured file is already open for read
> > > */
> > > + if ((mask == MAY_WRITE) || (mask == MAY_APPEND)) {
> > > + int mask_sav = data->mask;
> > > + int rc;
> > > +
> > > + data->mask = MAY_READ;
> > > + rc = ima_must_measure(&idata);
> > > + if (!rc) {
> > > + if (atomic_read(&(data->dentry->d_count)) - 1 >
> > > + atomic_read(&(inode->i_writecount)))
> > > + ima_add_violation(inode, data->filename,
> > > + "invalid_pcr", "ToMToU");
> > > + }
> > > + data->mask = mask_sav;
> > > + goto out;
> > > + }
> >
> > Following up on Christoph's comment...
> >
> > I'm worried that this calculation isn't very precise. The calculation
> > that you're trying to come up with here is the number of opens (d_count)
> > vs. the number of writers (i_writecount). When they don't match, you
> > know that the new open is the first write, and you must 'invalidate the
> > PCR'?
> >
> > There are a number of things that elevate d_count, and it is a lot more
> > than just an open() that can do it. Is that OK?
>
> >From an integrity perspective, a file measurement might be invalidated
> unnecessarily, but it is safe. For any file when opened for write, while
> having an existing reader, will cause the file measurement to be
> invalidated. Can you give examples of things, other than open(), that
> elevate d_count?

Just do a little search for dget(). There are plenty of places that
we'll do a lookup (and a dget), realize that we can't complete an
operation (say O_RDWR on a 400 file), then dput() the dentry. If you
look at dput at the wrong time, you could mistake that for an *actual*
open.

Are you holding any locks here? If not, it is completely conceivable
that you do the atomic_read(d_count)=100, then by the time you do the
aomtic_read(i_writecount), 90 of those references have been dput()'d.

> Is there a different, better way to determine if there are any readers?

I think you're looking at it from the wrong angle. If you have a
writer, does it matter whether there are any readers? You should just
unconditionally invalidate the integrity measurement.

-- Dave