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

[Yongji Xie]

On 2016/5/5 20:15, Tian, Kevin 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...

Do you mean we can trigger MSIs that correspond to interrupt
IDs of other devices by writing to MSI-X table although IRQ
remapping is enabled?

On PPC64, there is a mapping between MSIs and PE num
which can be used to identify a PCI device on PHB. So the
hardware can ensure a given pci device can only shoot the
MSIs assigned for it.  Isn't there a similar mapping in IRQ
remapping table on Intel.

Thanks,
Yongji

> 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.
>
> Thanks
> Kevin