[GIT PULL] Integrity: IMA FUSE fixes

[Mimi Zohar]

On Sat, 2018-02-10 at 20:50 -0800, Linus Torvalds wrote:
> On Sat, Feb 10, 2018 at 8:41 PM, Mimi Zohar <zohar at linux.vnet.ibm.com> wrote:
> >>
> >> 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.
> > 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.
> 
> Failing seems like a sane model, although I also suspect it would just
> break a lot of cases that currently work fine because *in*practice*
> fuse works fine as a normal filesystem (think fuse "exfat" module
> etc).
> 
> So yes, the failing behavior is sane, but I agree with you that it
> should be something that requires a specific policy ("fail on
> untrusted filesystems like fuse").

Could we differentiate between untrusted from unprivileged and
untrusted fuse?  The existing fuse would continue to work, but on
systems with IMA-appraisal enabled the new, unprivileged fuse would
fail.

> But regardless, disabling caching just seems broken in all situations
> and never right, so I really don't want to pull that tree unless
> somebody can point out where it makes sense.

Agreed.  Re-measuring/appraising the file would only detect well
behaved malicious fuse.

Mimi

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