Re: [PATCH v3 1/3] uio: add ioctl support

[Michael S. Tsirkin]

On Tue, Oct 06, 2015 at 12:43:45AM +0300, Vladislav Zolotarov wrote:
> So, like it has already been asked in a different thread I'm going to
> ask a rhetorical question: what adding an MSI and MSI-X interrupts support to
> uio_pci_generic has to do with security?

memory protection is a better term than security.

It's very simple: you enable bus mastering and you ask userspace to map
all device BARs. One of these BARs holds the address to which device
writes to trigger MSI-X interrupt.

This is how MSI-X works, internally: from the point of view of
PCI it's a memory write. It just so happens that the destination
address is in the interrupt controller, that triggers an interrupt.

But a bug in this userspace application can corrupt the MSI-X table,
which in turn can easily corrupt kernel memory, or unrelated processes's
memory.  This is in my opinion unacceptable.

So you need to be very careful
- probably need to reset device before you even enable bus master
- prevent userspace from touching msi config
- prevent userspace from moving BARs since msi-x config is within a BAR
- detect reset and prevent linux from touching device while it's under
  reset

The list goes on and on.

This is pretty much what VFIO spent the last 3 years doing, except VFIO
also can do IOMMU groups.

> What "security threat" does it add
> that u don't already have today?

Yes, userspace can create this today if it tweaks PCI config space to
enable MSI-X, then corrupts the MSI-X table.  It's unfortunate that we
don't yet prevent this, but at least you need two things to go wrong for
this to trigger.

The reason, as I tried to point out, is simply that I didn't think
uio_pci_generic will be used for these configurations.
But there's nothing fundamental here that makes them secure
and that therefore makes your patches secure as well.

Fixing this to make uio_pci_generic write-protect MSI/MSI-X enable
registers sounds kind of reasonable, this shouldn't be too hard.

-- 
MST
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