linux-kernel - Re: [PATCH v2] KVM: kvm_io_bus_unregister

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Date:   Fri, 24 Mar 2017 10:33:55 +0100
From:   Cornelia Huck <cornelia.huck@...ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        KVM list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        stable <stable@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: kvm_io_bus_unregister_dev() should never fail

On Fri, 24 Mar 2017 09:55:15 +0100
David Hildenbrand <david@...hat.com> wrote:

> 
> >>> -             return r;
> >>> +     if (i == bus->dev_count)
> >>> +             return;
> >>>
> >>>       new_bus = kmalloc(sizeof(*bus) + ((bus->dev_count - 1) *
> >>>                         sizeof(struct kvm_io_range)), GFP_KERNEL);
> >>> -     if (!new_bus)
> >>> -             return -ENOMEM;
> >>> +     if (!new_bus)  {
> >>> +             pr_err("kvm: failed to shrink bus, removing it completely\n");
> >>> +             goto broken;
> >>
> >> The guest will fail in mysterious ways, if you do this (and
> >> io_bus_unregister_dev can be called during runtime): in-kernel device
> >> accesses will fail with unknown behaviour in the guest.
> 
> Actually, the next access to the BUS should result in -ENOMEM. And the
> error message should be enough to then figure out what went wrong.

Hopefully, an admin will look at the logs :)

But yes, the patch should have caught all issues in the host, and the
guest will basically be presented with broken "hardware".

> However, to hit this scenario at all feels very unlikely. So I would
> like to avoid advanced allocation schemes.

Agreed, spending too much time on complex recovery scenarios is
overkill for this unlikely case.