Date:   Tue, 12 Oct 2021 01:44:10 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Hetzelt, Felicitas" <f.hetzelt@...berlin.de>,
        "kaplan, david" <david.kaplan@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH 0/9] More virtio hardening

On Tue, Oct 12, 2021 at 10:43:57AM +0800, Jason Wang wrote:
> On Mon, Oct 11, 2021 at 8:36 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Mon, Oct 11, 2021 at 03:36:51PM +0800, Jason Wang wrote:
> > > On Tue, Oct 5, 2021 at 3:42 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Mon, Sep 13, 2021 at 01:53:44PM +0800, Jason Wang wrote:
> > > > > Hi All:
> > > > >
> > > > > This series treis to do more hardening for virito.
> > > > >
> > > > > patch 1 validates the num_queues for virio-blk device.
> > > > > patch 2-4 validates max_nr_ports for virito-console device.
> > > > > patch 5-7 harden virtio-pci interrupts to make sure no exepcted
> > > > > interrupt handler is tiggered. If this makes sense we can do similar
> > > > > things in other transport drivers.
> > > > > patch 8-9 validate used ring length.
> > > > >
> > > > > Smoking test on blk/net with packed=on/off and iommu_platform=on/off.
> > > > >
> > > > > Please review.
> > > > >
> > > > > Thanks
> > > >
> > > > So I poked at console at least, and I think I see
> > > > an issue: if interrupt handler queues a work/bh,
> > > > then it can still run while reset is in progress.
> > >
> > > Looks like a bug which is unrelated to the hardening?
> >
> > Won't preventing use after free be relevant?
> 
> Oh right.
> 
> > I frankly don't know what does hardening means then.
> > > E.g the driver
> > > should sync with work/bh before reset.
> >
> > No, there's no way to fix it ATM without extra locks and state which I
> > think we should strive to avoid or make it generic, not per-driver,
> > since sync before reset is useless, new interrupts will just arrive and
> > queue more work. And a sync after reset is too late since driver will
> > try to add buffers.
> 
> Can we do something like
> 
> 1) disable interrupt
> 2) sync bh
> 
> Or I guess this is somehow you meant in the following steps.

So that would mean a new API to disable vq interrupts.
reset will re-enable.
E.g. virtqueue_cancel_cb_before_reset()?

Then drivers can sync, then reset.
This means maintaining more state though, which I don't like.

An alternative is something like this:

static void (*virtio_flush_device)(struct virtio_device *dev);

void virtio_reset_device(struct virtio_device *dev, virtio_flush_device flush)
{
	might_sleep();
	if (flush) {
		dev->config->disable_interrupts(dev);
		flush(dev);
		dev->config->reset(dev);
		dev->config->enable_interrupts(dev);
	} else {
		dev->config->reset(dev);
	}
}

I have patches wrapping all reset calls in virtio_reset_device
(without the flush parameter but that's easy to tweak).


> >
> > Maybe we can break device. Two issues with that
> > - drivers might not be ready to handle add_buf failures
> > - restore needs to unbreak then and we don't have a way to do that yet
> >
> > So .. careful reading of all device drivers and hoping we don't mess
> > things up even more ... here we come.
> 
> Yes.

The biggest issue with this trick is drivers not handling add_buf
errors, adding a failure path here risks creating memory leaks.
OTOH with e.g. bounce buffers maybe it's possible for add buf to
fail anyway?

> >
> > > >
> > > > I sent a patch to fix it for console removal specifically,
> > > > but I suspect it's not enough e.g. freeze is still broken.
> > > > And note this has been reported without any TDX things -
> > > > it's not a malicious device issue, can be triggered just
> > > > by module unload.
> > > >
> > > > I am vaguely thinking about new APIs to disable/enable callbacks.
> > > > An alternative:
> > > >
> > > > 1. adding new remove_nocb/freeze_nocb calls
> > > > 2. disabling/enabling interrupts automatically around these
> > > > 3. gradually moving devices to using these
> > > > 4. once/if all device move, removing the old callbacks
> > > >
> > > > the advantage here is that we'll be sure calls are always
> > > > paired correctly.
> > >
> > > I'm not sure I get the idea, but my feeling is that it doesn't
> > > conflict with the interrupt hardening here (or at least the same
> > > method is required e.g NO_AUTO_EN).
> > >
> > > Thanks
> >
> > Right.  It's not that it conflicts, it's that I was hoping that
> > since you are working on hardening you can take up fixing that.
> > Let me know whether you have the time. Thanks!
> 
> I can do that.
> 
> Thanks
> 
> >
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > > Jason Wang (9):
> > > > >   virtio-blk: validate num_queues during probe
> > > > >   virtio: add doc for validate() method
> > > > >   virtio-console: switch to use .validate()
> > > > >   virtio_console: validate max_nr_ports before trying to use it
> > > > >   virtio_config: introduce a new ready method
> > > > >   virtio_pci: harden MSI-X interrupts
> > > > >   virtio-pci: harden INTX interrupts
> > > > >   virtio_ring: fix typos in vring_desc_extra
> > > > >   virtio_ring: validate used buffer length
> > > > >
> > > > >  drivers/block/virtio_blk.c         |  3 +-
> > > > >  drivers/char/virtio_console.c      | 51 +++++++++++++++++++++---------
> > > > >  drivers/virtio/virtio_pci_common.c | 43 +++++++++++++++++++++----
> > > > >  drivers/virtio/virtio_pci_common.h |  7 ++--
> > > > >  drivers/virtio/virtio_pci_legacy.c |  5 +--
> > > > >  drivers/virtio/virtio_pci_modern.c |  6 ++--
> > > > >  drivers/virtio/virtio_ring.c       | 27 ++++++++++++++--
> > > > >  include/linux/virtio.h             |  1 +
> > > > >  include/linux/virtio_config.h      |  6 ++++
> > > > >  9 files changed, 118 insertions(+), 31 deletions(-)
> > > > >
> > > > > --
> > > > > 2.25.1
> > > >
> >

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