linux-kernel - Re: [GIT PULL] Integrity: IMA FUSE fixes

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Date:   Mon, 12 Feb 2018 00:19:55 +1100 (AEDT)
From:   James Morris <jmorris@...ei.org>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>
cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [GIT PULL] Integrity: IMA FUSE fixes

On Sat, 10 Feb 2018, Mimi Zohar wrote:

> > Which seems *worse* than what we do now, in that it wastes time and
> > effort on re-creating those pointless measurements because it disables
> > the caching of them.
> > 
> > So honestly, the only sane thing seems to be to disable IMA on fuse,
> > not to force it to do even _more_ pointless work.
> > 
> > What am I missing?
> 
> No, you're right.  The file could change at any time, making the
> measurement(s) and by extension signature verification meaningless. 

Really?  I thought the whole idea of IMA was that it only detects offline 
tampering and it specifically does not protect against runtime attacks.

Any file could change after measurement on a compromised or misconfigured 
system.  e.g. via direct write to the block device.

> Custom policy rules could be defined to disable measurement,
> appraisal, and audit for files on fuse.  However, I don't think we
> want to automatically disable measurement, even meaningless
> measurements.  Some indication needs to be included for remote
> attestation, security analytics, or forensics.  For systems with
> policies that require file signatures even on fuse, the safest thing
> would seem to be to fail the signature verification.

I don't understand this -- if a file passes signature verification, it 
passes.  If it was modified and still passes, the problem is elsewhere.

I don't think the FUSE measurements are inherently useless, or more 
useless than any others, at least.  You can misconfigure all kinds of 
things on a system which would undermine IMA, and I would count allowing 
unprivileged use of FUSE with critical files as such.

The point of the patches, IIUC, was that FUSE had no useful way to notify 
IMA that a file had changed, so, always measure.  IMA assumes that changes 
to a running system are made under the control of a correctly enforced 
security policy.  If you're using FUSE and IMA, then you should understand 
the security implications of that.  Or am I missing something?

- James
-- 
James Morris
<jmorris@...ei.org>