linux-kernel - Re: [dm-devel] [PATCH] dm: add secdel target

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Date:   Fri, 19 Oct 2018 15:11:21 +0300
From:   Vitaly Chikunov <vt@...linux.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        Jonathan Corbet <corbet@....net>, Shaohua Li <shli@...nel.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-raid@...r.kernel.org
Subject: Re: [dm-devel] [PATCH] dm: add secdel target

On Fri, Oct 19, 2018 at 05:00:33AM -0700, Christoph Hellwig wrote:
> On Fri, Oct 19, 2018 at 02:49:44PM +0300, Vitaly Chikunov wrote:
> > On Thu, Oct 18, 2018 at 11:19:45PM -0700, Christoph Hellwig wrote:
> > > Just as a note:  the name is a complete misowner, a couple overwrite
> > > are not in any way secure deletion.  So naming it this way and exposing
> > > this as erase is a problem that is going to get back to bite us.
> > 
> > In what way it's not secure deletion?
> > 
> > It's secure deletion by overwriting discarded data instead of leaving it
> > as is.
> 
> Overwriting data does not delete data.  Most certainly not in Flash based
> SSDs, but also not in many storage arrays, or for that matter many modern
> disks that have sectore remapping and various kinds of non-volatile
> caches.  There is a reason why devices tend to have special commands to
> perform secure erase - depending on the media they might or might not
> overwrite internally, but at least they do it in a way that actually
> works for the given media and device configuration.

I know that. This is why it says "The target does not try to determine
if the underlying drive reliably support data overwrites, this decision
is solely on the discretion of a user. Please note that not all drivers
support this ability."


> > dm-erase or dm-wipe? dm-discerase?
> 
> dm-overwrite?

These are all good to me.

> 
> > But still provide REQ_OP_SECURE_ERASE support?
> 
> On the one hand that is highly misleading and would warrant a warning
> (see above), on the other hand discard is purely advisory and can be
> skipped any time, including by intermediate layers.  So I don't think
> you can actually do what you want without major changes to the whole
> I/O stack.

Probably, a concerned user should test his setup to be sure discards
reach dm-secdel (after that they go as writes), and data he thinks
should be erased is erased.