[RFC PATCH] ima: force the re-evaluation of files on untrusted file systems

[zohar@linux.vnet.ibm.com (Mimi Zohar)]

On Mon, 2018-02-05 at 17:12 +0100, Miklos Szeredi wrote:
> On Mon, Feb 5, 2018 at 4:50 PM, James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
> > On Mon, 2018-02-05 at 08:40 -0500, Mimi Zohar wrote:
> >> On filesystems, such as fuse or remote filesystems, that we can not
> >> detect or rely on the filesystem to tell us when a file has changed,
> >> always re-measure, re-appraise, and/or re-audit the file.
> >
> > Using the presence or absence of d_revalidate isn't definitive for
> > uncacheable appraisals: all stacked filesystems have to implement
> > d_revalidate just in case the underlying has it, but it doesn't mean
> > their appraisals can't be cached if they're fully built on top of
> > traditional filesystems (like they are in the Docker/OCI use case).  I
> > think the original flag approach is better.  The only thing stackable
> > filesystems argues for is that for them it should probably be a
> > superblock flag so it can be per-mount point (depending on overlay
> > composition).
> >
> > d_revalidate() also strikes me as wrong from the semantic point of
> > view: it's about whether the path name to inode cache needs re-
> > evaluating not whether the underlying inode could change arbitrarily.
> >  These are definitely related but not necessarily equivalent concepts.
> 
> True.  A more precise indication is whether cache pages have been
> invalidated for a certain inode.   Can we used that?  I.e.
> invalidate_inode_pages*() calls down into IMA or sets a flags or
> whatever to indicate that the file contents might have changed.

I don't think that works for the FUSE use case.

Mimi

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