linux-kernel - Re: [PATCH RFC 3/8] fs/ext4: Disallow encryption if inode is DAX

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Date:   Wed, 15 Apr 2020 10:27:30 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     "Weiny, Ira" <ira.weiny@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dave Chinner <david@...morbit.com>,
        Christoph Hellwig <hch@....de>, Jeff Moyer <jmoyer@...hat.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH RFC 3/8] fs/ext4: Disallow encryption if inode is DAX

On Wed, Apr 15, 2020 at 9:03 AM Theodore Y. Ts'o <tytso@....edu> wrote:
>
> On Mon, Apr 13, 2020 at 09:00:25PM -0700, ira.weiny@...el.com wrote:
> > From: Ira Weiny <ira.weiny@...el.com>
> >
> > Encryption and DAX are incompatible.  Changing the DAX mode due to a
> > change in Encryption mode is wrong without a corresponding
> > address_space_operations update.
> >
> > Make the 2 options mutually exclusive by returning an error if DAX was
> > set first.
> >
> > Signed-off-by: Ira Weiny <ira.weiny@...el.com>
>
> The encryption flag is inherited from the containing directory, and
> directories can't have the DAX flag set, so anything we do in
> ext4_set_context() will be safety belt / sanity checking in nature.
>
> But we *do* need to figure out what we do with mount -o dax=always
> when the file system might have encrypted files.  My previous comments
> about the verity flag and dax flag applies here.
>
> Also note that encrypted files are read/write so we must never allow
> the combination of ENCRPYT_FL and DAX_FL.  So that may be something
> where we should teach __ext4_iget() to check for this, and declare the
> file system as corrupted if it sees this combination.  (For VERITY_FL
> && DAX_FL that is a combo that we might want to support in the future,
> so that's probably a case where arguably, we should just ignore the
> DAX_FL for now.)

We also have a pending consideration for what MKTME (inline memory
encryption with programmable hardware keys) means for file-encryption
+ dax. Certainly kernel based software encryption is incompatible with
dax, but one of the hallway track discussions I wanted to have at LSF
is whether fscrypt is the right interface for managing inline memory
encryption. For now, disallowing ENCRPYT_FL + DAX_FL seems ok if
ENCRPYT_FL always means software encryption, but this is something to
circle back to once we get MKTME implemented for volume encryption and
start to look at finer grained (per-directory key) encryption.