Re: [Qemu-devel] [PATCH 0/3] vpc: support probing of fixed size images

[Markus Armbruster]

Kevin Wolf <address@hidden> writes:

> Am 14.08.2014 um 16:57 hat Jeff Cody geschrieben:
>> On Thu, Aug 14, 2014 at 10:42:27AM -0400, Levente Kurusa wrote:
>> > On Tuesday, 12 August, 2014 3:35:42 PM, Jeff Cody wrote:
>> > > On Tue, Aug 12, 2014 at 02:20:34PM +0100, Stefan Hajnoczi wrote:
>> > > > On Fri, Aug 01, 2014 at 03:39:58PM +0200, Levente Kurusa wrote:
>> > > > > Fixed size VPC images do not have a footer, hence the current probe
>> > > > > function will fail and QEMU will fall back to the raw_bsd driver, 
>> > > > > which
>> > > > > is
>> > > > > not the correct behaviour. The specification of the format says that
>> > > > > fixed
>> > > > > size images have a footer as the last 512 bytes of the file. The 
>> > > > > footer
>> > > > > is
>> > > > > exactly the same as the header would be in the case of dynamically
>> > > > > growing
>> > > > > images.
>> > > > > 
>> > > > > For this, we need to read the last 512 bytes of the image, however 
>> > > > > the
>> > > > > current mechanics predominantly read the first 2048 bytes and pass 
>> > > > > that
>> > > > > as a buffer to the probe functions. Solve this by passing the
>> > > > > BlockDriverState to the probe functions, hence giving them a chance 
>> > > > > to
>> > > > > read
>> > > > > the extra bytes they might need.
>> > > > 
>> > > > I hesitate to add patches that extend image format probing.  For the
>> > > > past few years we have always recommended that image files should not 
>> > > > be
>> > > > probed.
>> > > > 
>> > > > Image probing is prone to security issues because a malicious guest can
>> > > > modify a raw or vpc image by putting another image format header at
>> > > > sector 0.  The next time QEMU opens the image it will detect a 
>> > > > different
>> > > > format.  One evil trick is to refer to a file on the host file system 
>> > > > as
>> > > > the backing file, now you can read any file that the QEMU process has
>> > > > access to.
>> > 
>> > Yea, good point. The current state of probing is actually quite bad,
>> > just take a look at dmg_probe in block/dmg.c :-(
>> > 
>> > > > 
>> > > > Probing also complicates live migration.  The source host still has the
>> > > > image file open and may write to it.  The destination host shouldn't
>> > > > even read from the image file before handover to avoid file cache
>> > > > coherency issues.
>> > > > 
>> > > > Probing is broken.  It shouldn't be used.  We shouldn't extend it
>> > > > (especially by adding more I/Os).
>> > 
>> > Even though, my series would have only added one extra I/O in the case
>> > of failing VPC images, I have to admit you are right.
>> > 
>> > > 
>> > > For 2.2, maybe we should limit probing to only certain operations (e.g.
>> > > qemu-img info) - or perhaps just remove the capability altogether, or
>> > > at least start phasing it out with a warning message that automatic
>> > > format detection is deprecated and may be unsafe.
>> > > 
>> > 
>> > Considering the fact that most open functions already check the magic
>> > numbers, and they do a lot better/safer job at it, we could just swap
>> > the probe functions with the open ones and just insert an fprintf
>> > when format is not specified doing what Jeff suggested.
>> > 
>> > Any objections to this?
>> > 
>> > (This would also solve the VPC-fixed-size bug, since vpc_open already
>> > checks the footer if the header is not found)
>> >
>> 
>> I was proposing actually going a bit further than this, and not
>> allowing automatic format detection at all, with an exception for
>> 'qemu-img info'. In the interim, until that is in place and it is
>> removed, warn with a deprecation message.
>
> No, we can't do this. It would immediately destroy -hda and similar
> convenience options and make the command line really hard to use even
> for simple cases. I usually call qemu manually and I specify format
> basically _never_, because it would like double the length of my command
> line (okay, not quite, but my command lines are usually very short) and
> I know what I'm doing and I'm running trusted guests.
>
> Plus, there are probably many scripts out there that rely on it.
>
> A more reasonable approach would be to just forbid probing raw and
> raw-like formats like VHD fixed (the rest should be safe), but I think
> the impact of this would still be too bad.

I think we're doing our users a disservice by sticking to the fatally
flawed probing.  "Broken by default" is just wrong, and "convenience" is
no excuse.

I believe we can retain 90% of the convenience without the flaws, by
defaulting format based on file meta-data only: name and struct stat.

This breaks down for things like QCOW2-formatted logical volumes.  But
those things are exactly *not* the things casual users need.

It also breaks down when users call their QCOW2 images foo.vmdk, but I'd
call that a feature :)