Re: [Xen-devel] [PATCH v3 10/10] x86/MSI-X: provide hypercall interface for mask-all control

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Jun 12, 2015 at 02:51:02PM +0100, Jan Beulich wrote:
> >>> On 12.06.15 at 15:21, <konrad.wilk@xxxxxxxxxx> wrote:
> > On Thu, Jun 11, 2015 at 09:35:51AM +0100, Jan Beulich wrote:
> >> >>> On 05.06.15 at 13:28, <JBeulich@xxxxxxxx> wrote:
> >> > Qemu shouldn't be fiddling with this bit directly, as the hypervisor
> >> > may (and now does) use it for its own purposes. Provide it with a
> >> > replacement interface, allowing the hypervisor to track host and guest
> >> > masking intentions independently (clearing the bit only when both want
> >> > it clear).
> >> 
> >> Originally I merely meant to ping the tools side changes here
> >> (considering that the original issue has been pending for months,
> >> delayed by various security issues as well as slow turnaround on
> >> understanding the nature and validity of that original issue, I'd
> >> _really_ like to see this go in now), but thinking about it once
> >> again over night I realized that what we do here to allow qemu
> >> to be fixed would then also be made use of by the kernels
> >> running pciback: While Dom0 fiddling with the MSI-X mask-all bit
> >> for its own purposes is at least not a security problem, it doing
> >> so on behalf of (and directed by) a guest would be as soon as
> >> the hypervisor side patches making use of that bit went in.
> > 
> > It is hard to comment on this since I don't know exactly what
> > those patches would do.
> 
> Did you take a look?

No. Oddly enough they didn't show up in my thread and I didn't
even look at the title to Google for it. Doing it now.
> 
> >  But the 'pci_msi_ignore_mask'
> > from 38737d82f9f0168955f9944c3f8bd3bb262c7e88, "PCI/MSI: Add
> > pci_msi_ignore_mask to prevent writes to MSI/MSI-X Mask Bits""
> > should have prevented that. That said said patches could change
> > the pci_msi_ignore_mask of course.
> 
> For one, this doesn't deal with the MSI-X mask-all bit. And then it
> only suppresses functionality that the guest really ought to be
> allowed to use, just not by directly manipulating hardware. Plus
> of course any older Linux as well as other OSes would still be a
> problem.

True.
> 
> Jan
> 

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