Re: [PATCH 1/3] VFS: Add a call to obtain a file's hash

[Mimi Zohar <zohar@linux.ibm.com>]

On Thu, 2018-10-11 at 11:24 -0700, Matthew Garrett wrote:
> On Thu, Oct 11, 2018 at 11:21 AM Matthew Garrett <mjg59@google.com> wrote:
> >
> > On Thu, Oct 11, 2018 at 8:22 AM Mimi Zohar <zohar@linux.ibm.com> wrote:
> > >
> > > This patch description starts out saying that IMA needs the file hash
> > > without explaining why.  Without that explanation, simply extracting
> > > the file hash included in the file signature might sound plausible,
> > > but kind of defeats the purpose of IMA.
> >
> > I'm not sure how it defeats the purpose - IMA wants to know the hash
> > of a file so it can either log it or compare it against a signature,
> > and it currently obtains this hash by reading the entire file at
> > measurement time. If the filesystem later returns different data then
> > IMA won't notice, which allows a malicious filesystem to bypass the
> > measurements - there's no guarantee that we won't evict large parts of
> > the copy of an executable that IMA read, and the filesystem can give
> > us back a modified page when we page it back in. So IMA fundamentally
> > relies on the filesystem to be trustworthy, and if we rely on the
> > filesystem to be trustworthy then we should be able to rely on it to
> > accurately store and provide the hash of a file.
> 
> Oh, to clarify on the signature part of things - it would obviously be
> inappropriate to, say, just read the hash out of security.ima and hand
> that back.

Right, reading it either directly or extracted from the file signature
stored in security.ima.

> But for a hypothetical case where the filesystem itself
> verifies the signature, then the filesystem would abort the
> transaction if the signature didn't match and it seems reasonable to
> avoid doing the validation twice (once up front and then again on
> every read)

Right, this is a hypothetical scenario as far as I'm aware, since none
of the filesystems are currently calculating and storing the file
hash.  The default should be for IMA to re-calculate the file hash.

Mimi