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Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults |
Date: |
Thu, 14 May 2015 15:57:15 +0200 |
On Thu, May 14, 2015 at 03:25:39PM +0200, Sander Eikelenboom wrote:
>
> Thursday, May 14, 2015, 2:53:17 PM, you wrote:
>
>
>
> > On 14/05/2015 14:45, Markus Armbruster wrote:
> >> Paolo Bonzini <address@hidden> writes:
> >>
> >>> On 14/05/2015 14:02, Markus Armbruster wrote:
> >>>> It should certainly be off for pc-q35-2.4 and newer. Real Q35 boards
> >>>> commonly don't have an FDC (depends on the Super I/O chip used).
> >>>>
> >>>> We may want to keep it off for pc-i440fx-2.4 and newer. I doubt
> >>>> there's a real i440FX without an FDC, but our virtual i440FX is quite
> >>>> unlike a real one in other ways already.
> >>>
> >>> That would break libvirt for people upgrading from 2.3 to 2.4. So it's
> >>> more like pc-i440fx-3.0 and pc-q35-3.0.
> >>
> >> What exactly breaks when?
>
> > libvirt expects "-nodefaults -drive if=none,id=fdd0,... -global
> > isa-fdc.driveA=fdd0" to result in a machine with a working FDD. It
> > doesn't know that it has to add "-machine fdc=on".
>
> > Besides, adding a new machine option is not the best we can do. If the
> > default is "no FDC", all that is needed to add one back is -device. An
> > FDC is yet another ISA device, it is possible to create one with -device.
>
> >> add the magic to make -global isa-fdc... auto-set the option to on.
>
> > That would be ugly magic.
>
> > The more I think about this, the more I think this is just a kneejerk
> > reaction to a sensationalist announcement. The effect of this
> > vulnerability on properly configured data centers (running
> > non-prehistoric versions of Xen or KVM and using
> > stubdom/SELinux/AppArmor properly) should be really close to zero.
>
> > It's a storm in a tea cup.
>
> > Paolo
I agree. An option to disable fdc sounds like a reasonable thing to do
reduce attack surface while keeping full compatibility. After all
downstreams disable devices they don't want to support, too. Changing
defaults just because a device had a bug does sound extreme.
> I tend to kindly disagree if you look at the broader perspective, yes it's
> could
> be a storm in a tea cup, but there seems to be a pattern.
>
> From a "cmdline user" / "platform emulation" point of view i can imagine that
> some sort of
> auto configuration / bundling in platforms is necessary (to limit the length
> of
> the cmdline to be supplied).
>
> But from a library / toolstack point of view it's quite nasty if *all* more
> or
> less obscure options have to actively disabled. It's quite undoable, not to
> mention what
> happens when new ones get added.
I support this statement for new features. Users should opt-in to them.
E.g. we learned this the hard way with pvpanic.
> From this PoV it would be better to have to actively enable all the stuff you
> want.
>
> At least Xen seemed to have taken the "no-defaults" as the best option to get
> there so far, but that doesn't seem to
>
> It's not the first CVE that has come from this "you have to actively disable
> all
> you don't want to happen" and probably won't be the last.
>
> So a "-no-defaults-now-for-real" option/mode for libraries / toolstacks, that
> requires them to be very verbose, explicit and specific in what they actually
> want, could be valuable.
>
> --
> Sander
If you would change what this flag means in each release, that would
achieve no useful purpose IMO.
> >>> Unless for q35 we decide to
> >>> break everything and retroactively nuke the controller.
> >>>
> >>> (I'm still not sure why we have backwards-compatible machine types for
> >>> q35).
> >>
> >> Beats me :)
> >>
> >> [...]
> >>
>
I'm fine with making them all identical.
--
MST
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, (continued)
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Markus Armbruster, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Daniel P. Berrange, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Sander Eikelenboom, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Daniel P. Berrange, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Stefano Stabellini, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Stefano Stabellini, 2015/05/14
- Re: [Qemu-devel] [Xen-devel] [PATCH] Do not emulate a floppy drive when -nodefaults,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Michael S. Tsirkin, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, John Snow, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Markus Armbruster, 2015/05/15
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/15
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Stefano Stabellini, 2015/05/15
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Kevin Wolf, 2015/05/18
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Michael S. Tsirkin, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Paolo Bonzini, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Michael S. Tsirkin, 2015/05/14
- Re: [Qemu-devel] [PATCH] Do not emulate a floppy drive when -nodefaults, Markus Armbruster, 2015/05/14