linux-kernel - Re: [PATCH v2 1/3] dm-inlinecrypt: Add inline encryption support

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Message-ID: <20240924220434.GB1585@sol.localdomain>
Date: Tue, 24 Sep 2024 15:04:34 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: Christoph Hellwig <hch@...radead.org>
Cc: Md Sadre Alam <quic_mdalam@...cinc.com>, axboe@...nel.dk,
	song@...nel.org, yukuai3@...wei.com, agk@...hat.com,
	snitzer@...nel.org, mpatocka@...hat.com, adrian.hunter@...el.com,
	quic_asutoshd@...cinc.com, ritesh.list@...il.com,
	ulf.hansson@...aro.org, andersson@...nel.org,
	konradybcio@...nel.org, kees@...nel.org, gustavoars@...nel.org,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, dm-devel@...ts.linux.dev,
	linux-mmc@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-hardening@...r.kernel.org, quic_srichara@...cinc.com,
	quic_varada@...cinc.com
Subject: Re: [PATCH v2 1/3] dm-inlinecrypt: Add inline encryption support

On Tue, Sep 24, 2024 at 12:44:53AM -0700, Christoph Hellwig wrote:
> On Sat, Sep 21, 2024 at 11:55:19AM -0700, Eric Biggers wrote:
> > (https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/drivers/md/dm-default-key.c),
> > and I've been looking for the best way to get the functionality upstream.  The
> > main challenge is that dm-default-key is integrated with fscrypt, such that if
> > fscrypt encrypts the data, then the data isn't also encrypted with the block
> > device key.  There are also cases such as f2fs garbage collection in which
> > filesystems read/write raw data without en/decryption by any key.  So
> > essentially a passthrough mode is supported on individual I/O requests.
> 
> Adding a default key is not the job of a block remapping driver.  You'll
> need to fit that into the file system and/or file system level helpers.

What about a block device ioctl, as was previously proposed
(https://lore.kernel.org/linux-block/1658316391-13472-1-git-send-email-israelr@nvidia.com/T/#u)?

> > It looks like this patch not only does not support that, but it ignores the
> > existence of fscrypt (or any other use of inline encryption by filesystems)
> > entirely, and overrides any filesystem-provided key with the block device's.  At
> > the very least, this case would need to be explicitly not supported initially,
> > i.e. dm-inlinecrypt would error out if the upper layer already provided a key.
> 
> I agree that we have an incompatibility here, but simply erroring out
> feels like the wrong way to approach the stacking.  If a stacking driver
> consumes the inline encryption capability it must not advertise it to
> the upper layers.

Right, I missed that's actually already how it works.  The crypto capabilities
are only passed through if the target sets DM_TARGET_PASSES_CRYPTO.

- Eric