linux-ext4 - Re: Proposal: Yet another possible fs-verity interface

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Date:   Tue, 12 Feb 2019 09:44:39 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        linux-fscrypt@...r.kernel.org,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: Proposal: Yet another possible fs-verity interface

> At that point, the merkle tree thing ends up fairly equivalent to
the
> > IMA "measurement" thing, with the exception that the filesystem *may*
> > optimize it to be long-term. Hmm?
> 
> Well, except that it's just a less efficient way of doing IMA
> "measurement" (if the file system doesn't support Merkle tree
> storage).
> 
> So adding that complexity is, in my view, not really worth it, since I
> very much doubt anyone would use a slower scheme.  I think the much
> better model is that fsverity is for file system where you can store
> the Merkle tree (and it's really not that hard for most file systems
> to store it); and if you are using a file system which doesn't, use
> IMA in good health.

Ted, one of the problems with IMA is that the file hash/signature
verification is at file open.  It isn't aware when files are brought
in from cache.  Does fs-verity re-verify blocks as they're restored
from cache?  For some use cases, that might justify the up front cost
associated with building the Merkle tree.

Mimi