linux-ext4 - Re: [PATCH v2 00/23] fs-verity support for XFS

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Date:   Wed, 12 Apr 2023 05:40:24 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Andrey Albershteyn <aalbersh@...hat.com>, djwong@...nel.org,
        dchinner@...hat.com, linux-xfs@...r.kernel.org,
        fsverity@...ts.linux.dev, rpeterso@...hat.com, agruenba@...hat.com,
        xiang@...nel.org, chao@...nel.org,
        damien.lemoal@...nsource.wdc.com, jth@...nel.org,
        linux-erofs@...ts.ozlabs.org, linux-btrfs@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-f2fs-devel@...ts.sourceforge.net,
        cluster-devel@...hat.com
Subject: Re: [PATCH v2 00/23] fs-verity support for XFS

On Tue, Apr 11, 2023 at 07:33:19PM -0700, Eric Biggers wrote:
> It seems it's really just the Merkle tree caching interface that is causing
> problems, as it's currently too closely tied to the page cache?  That is just an
> implementation detail that could be reworked along the lines of what is being
> discussed.

Well, that and some of the XFS internal changes that seem a bit ugly.

But it's not only very much tied to the page cache, but also to
page aligned data, which is really part of the problem.

> But anyway, it is up to the XFS folks.  Keep in mind there is also the option of
> doing what btrfs is doing, where it stores the Merkle tree separately from the
> file data stream, but caches it past i_size in the page cache at runtime.

That seems to be the worst of both worlds.

> I guess there is also the issue of encryption, which hasn't come up yet since
> we're talking about fsverity support only.  The Merkle tree (including the
> fsverity_descriptor) is supposed to be encrypted, just like the file contents
> are.  Having it be stored after the file contents accomplishes that easily...
> Of course, it doesn't have to be that way; a separate key could be derived, or
> the Merkle tree blocks could be encrypted with the file contents key using
> indices past i_size, without them physically being stored in the data stream.

xattrs contents better be encrypted as well, fsverity or not.