Re: [Qemu-devel] [PATCH 0/3] Remove artificial length limits when parsing options

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: qemu-devel@nongnu.org, Eduardo Habkost <ehabkost@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Marcel Apfelbaum <marcel@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Richard Henderson <rth@twiddle.net>
Subject: Re: [Qemu-devel] [PATCH 0/3] Remove artificial length limits when parsing options
Date: Mon, 16 Apr 2018 17:38:39 +0100	[thread overview]
Message-ID: <20180416163839.GC17600@redhat.com> (raw)
In-Reply-To: <87muy3m7uy.fsf@dusky.pond.sub.org>

On Mon, Apr 16, 2018 at 06:30:45PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > A user trying out SMBIOS "OEM strings" feature reported that the data
> > they are exposing to the guest was truncated at 1023 bytes, which breaks
> > the app consuming in the guest. After searching for the cause I
> > eventually found that the QemuOpts parsing is using fixed length 1024
> > byte array for option values and 128 byte array for key names.
> >
> > We can certainly debate whether it is sane to have such long command
> > line argument values (it is not sane), but if the OS was capable of
> > exec'ing QEMU with such an ARGV array, there is little good reason for
> > imposing an artificial length restriction when parsing it. Even worse is
> > that we silently truncate without reporting an error when hitting limits
> > resulting in a semantically incorrect behaviour, possibly even leading
> > to security flaws depending on the data that was truncated.
> >
> > Thus this patch series removes the artificial length limits by killing
> > the fixed length buffers.
> >
> > Separately I intend to make it possible to read "OEM strings" data from
> > a file, to avoid need to have long command line args.
> 
> Too bad I haven't been able to complete my quest to kill QemuOpts.
> 
> As far as I know, keyval.c's only arbitrary limit is the length of a key
> fragment (the things separated by '.').

Looks like that's the same scenario I tried to address in patch 2. The
'key' part in QemuOpts has the same 128 byte limit as in the keyval.c
code. I fear that could be hit with -blockdev when setting params on
very deeply nested block backends.  On the plus side  keyval.c actually
reports an error when it hits its 128 byte limit, instead of silently
carrying on as if all was well like QemuOpts did :-)

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|