Re: [PATCH v5 3/7] qcow: Tolerate backing_fmt=, but warn on backing_fmt=raw

[Eric Blake]

On 5/5/20 10:30 AM, Eric Blake wrote:
> On 5/5/20 2:35 AM, Kevin Wolf wrote:
> > Am 03.04.2020 um 19:58 hat Eric Blake geschrieben:
> > > qcow has no space in the metadata to store a backing format, and there
> > > are existing qcow images backed both by raw or by other formats
> > > (usually qcow) images, reliant on probing to tell the difference.
> > > While we don't recommend the creation of new qcow images (as qcow2 is
> > > hands-down better), we can at least insist that if the user does
> > > request a specific format without using -u, then it must be non-raw
> > > (as a raw backing file that gets inadvertently edited into some other
> > > format can form a security hole); if the user does not request a
> > > specific format or lies when using -u, then the status quo of probing
> > > for the backing format remains intact (although an upcoming patch will
> > > warn when omitting a format request).  Thus, when this series is
> > > complete, the only way to use a backing file for qcow without
> > > triggering a warning is when using -F if the backing file is non-raw
> > > to begin with.  Note that this is only for QemuOpts usage; there is no
> > > change to the QAPI to allow a format through -blockdev.
> > >
> > > Add a new iotest 290 just for qcow, to demonstrate the new warning.
> > >
> > > Signed-off-by: Eric Blake <eblake@redhat.com>
> >
> > Somehow this feels backwards. Not specifying the backing file format at
> > all isn't any safer than explicitly specifying raw.
> >
> > If there is a difference at all, I would say that explicitly specifying
> > raw means that the user is aware what they are doing. So we would have
> > more reason to warn against raw images if the backing format isn't
> > specified at all because then the user might not be aware that they are
> > using a backing file that probes as raw.
>
> Prior to this patch, -F does not work with qcow.  And even with this 
> patch, we still cannot store the explicit value of -F in the qcow file. 
> Anything that does not use -F must continue to work for now (although it 
> may now warn, and in fact must warn if we deprecate it), while anything 
> explicit is free to fail (since it failed already), but could also be 
> made to work (if letting it work is nicer than making it fail, and where 
> "work" may still include a warning, although it's pointless to have 
> something brand new that works but is deprecated out of the box).  So 
> the following is my summary of the two options we can choose between:
>
> Option 1, qcow backed by raw is more common than qcow backed by other, 
> so we want:
> raw <- qcow, no -F: work without warning (but if backing file is edited, 
> a future probe seeing non-raw would break image)
> raw <- qcow, with -F: work without warning (but if backing file is 
> edited, a future probe seeing non-raw would break image)
> other <- qcow, no -F: works but issues a warning (but backing file will 
> always probe correctly)
> other <- qcow, with -F: fails (we cannot honor the user's explicit 
> request, because we would still have to probe)
>
> Option 2, qcow backed by other is more common than qcow backed by raw, 
> so we want:
> raw <- qcow, no -F: works but issues a warning (using a raw backing file 
> without explicit buy-in is risky)
> raw <- qcow, with -F: works but issues a warning (explicit buy-in will 
> still require subsequent probing, and a backing file could change which 
> would break image)
> other <- qcow, no -F: works without warning
> other <- qcow, with -F: works without warning (later probing will still 
> see non-raw)
>
> It looks like you are leaning more towards option 1, while my patch 
> leaned more towards option 2.  Anyone else want to chime in with an 
> opinion on which is safer vs. easier?

> Option 3:
> completely deprecate qcow images with backing files, as there is no safe 
> way to do things favoring either raw (option 1) or non-raw (option 2), 
> and therefore accept -F solely for convenience with the rest of the 
> series, but always issue a warning regardless of whether -F was present.


Hearing no other opinion in the meantime, I've come up with option 4:

raw <- qcow, no -F: works but issues a warning to use -F (the user 
should be explicit that they know they are using raw)
raw <- qcow, with -F raw: a probe is attempted, if it returns anything 
other than raw, then fail (since we can't store the backing type, and 
the user's explicit type didn't match reality); otherwise works without 
warning (users tend to treat backing files as read-only, so even though 
editing a backing file could make it appear non-raw, that's less likely 
to happen)
other <- qcow, no -F: works without warning (we'll probe in future 
opens, but the probe will see the same file type and not corrupt user data)
other <- qcow, with -F: a probe is attempted and must match, but 
otherwise works without warning (we'll still have to probe in future 
opens, but it's no worse than before)



--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
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