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

From: Tian, Kevin
Date: Thu May 05 2016 - 08:15:56 EST

> 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.