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

[Miklos Szeredi]

On Mon, Feb 5, 2018 at 4:50 PM, James Bottomley
<James.Bottomley at 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.

Thanks,
Miklos
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