RE: [PATCH 5/5] vfio-pci: Allow to mmap MSI-X table if interrupt remapping is supported

[Tian, Kevin]

> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: Thursday, May 05, 2016 11:05 PM
> 
> On Thu, 5 May 2016 12:15:46 +0000
> "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> 
> > > From: Yongji Xie [mailto:xyjxie@xxxxxxxxxxxxxxxxxx]
> > > Sent: Thursday, May 05, 2016 7:43 PM
> > >
> > > Hi David and Kevin,
> > >
> > > On 2016/5/5 17:54, David Laight wrote:
> > >
> > > > From: Tian, Kevin
> > > >> Sent: 05 May 2016 10:37
> > > > ...
> > > >>> Acutually, we are not aimed at accessing MSI-X table from
> > > >>> guest. So I think it's safe to passthrough MSI-X table if we
> > > >>> can make sure guest kernel would not touch MSI-X table in
> > > >>> normal code path such as para-virtualized guest kernel on PPC64.
> > > >>>
> > > >> Then how do you prevent malicious guest kernel accessing it?
> > > > Or a malicious guest driver for an ethernet card setting up
> > > > the receive buffer ring to contain a single word entry that
> > > > contains the address associated with an MSI-X interrupt and
> > > > then using a loopback mode to cause a specific packet be
> > > > received that writes the required word through that address.
> > > >
> > > > Remember the PCIe cycle for an interrupt is a normal memory write
> > > > cycle.
> > > >
> > > > 	David
> > > >
> > >
> > > If we have enough permission to load a malicious driver or
> > > kernel, we can easily break the guest without exposed
> > > MSI-X table.
> > >
> > > I think it should be safe to expose MSI-X table if we can
> > > make sure that malicious guest driver/kernel can't use
> > > the MSI-X table to break other guest or host. The
> > > capability of IRQ remapping could provide this
> > > kind of protection.
> > >
> >
> > With IRQ remapping it doesn't mean you can pass through MSI-X
> > structure to guest. I know actual IRQ remapping might be platform
> > specific, but at least for Intel VT-d specification, MSI-X entry must
> > be configured with a remappable format by host kernel which
> > contains an index into IRQ remapping table. The index will find a
> > IRQ remapping entry which controls interrupt routing for a specific
> > device. If you allow a malicious program random index into MSI-X
> > entry of assigned device, the hole is obvious...
> >
> > Above might make sense only for a IRQ remapping implementation
> > which doesn't rely on extended MSI-X format (e.g. simply based on
> > BDF). If that's the case for PPC, then you should build MSI-X
> > passthrough based on this fact instead of general IRQ remapping
> > enabled or not.
> 
> I don't think anyone is expecting that we can expose the MSI-X vector
> table to the guest and the guest can make direct use of it.  The end
> goal here is that the guest on a power system is already
> paravirtualized to not program the device MSI-X by directly writing to
> the MSI-X vector table.  They have hypercalls for this since they
> always run virtualized.  Therefore a) they never intend to touch the
> MSI-X vector table and b) they have sufficient isolation that a guest
> can only hurt itself by doing so.
> 
> On x86 we don't have a), our method of programming the MSI-X vector
> table is to directly write to it. Therefore we will always require QEMU
> to place a MemoryRegion over the vector table to intercept those
> accesses.  However with interrupt remapping, we do have b) on x86, which
> means that we don't need to be so strict in disallowing user accesses
> to the MSI-X vector table.  It's not useful for configuring MSI-X on
> the device, but the user should only be able to hurt themselves by
> writing it directly.  x86 doesn't really get anything out of this
> change, but it helps this special case on power pretty significantly
> aiui.  Thanks,
> 

Allowing guest direct write to MSI-x table has system-wide impact.
As I explained earlier, hypervisor needs to control "interrupt_index"
programmed in MSI-X entry, which is used to associate a specific
IRQ remapping entry. Now if you expose whole MSI-x table to guest, 
it can program random index into MSI-X entry to associate with 
any IRQ remapping entry and then there won't be any isolation per se.

You can check "5.5.2 MSI and MSI-X Register Programming" in VT-d
spec.

Thanks
Kevin