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Am 22.07.20 um 09:19 schrieb Christian
König:
Am 22.07.20 um 02:22 schrieb
Gurchetan Singh:
Hi list,
virtio-gpu is moving in the direction where BO pages are
pinned for
the lifetime for simplicity. I am wondering if that is
considered a
security issue in general, especially after running into the
description of the new DMABUF_MOVE_NOTIFY config option.
Yes, that is generally considered a deny of service possibility
and so far Dave and Daniel have rejected all tries to upstream
stuff like this as far as I know.
DMA-buf an pinning for scanout are the only exceptions since the
implementation wouldn't have been possible otherwise.
Or better say for scanout pinning is a hardware requirement. For
DMA-buf we obviously can have a better approach :)
Christian.
Most drivers do not have a shrinker, or whether a BO is
purgeable is
entirely controlled by the userspace (madvice). They can be
categorized as "a security problem where userspace is able
to pin
unrestricted amounts of memory". But those drivers are
normally found
on systems without swap. I don't think the issue applies.
This is completely independent of the availability of swap or not.
Pinning of pages in large quantities can result in all kind of
problems and needs to be prevented even without swap.
Otherwise you can ran into problems even with simple I/O
operations for example.
Of the desktop GPU drivers, i915's shrinker certainly
supports purging
to swap. TTM is a bit hard to follow. I can't really tell
if amdgpu
or nouveau supports that. virtio-gpu is more commonly found
on
systems with swaps so I think it should follow the desktop
practices?
What we do at least in the amdgpu, radeon, i915 and nouveau is to
only allow it for scanout and that in turn is limited by the
physical number of CRTCs on the board.
Truth is, the emulated virtio-gpu device always supports
page moves
with VIRTIO_GPU_CMD_RESOURCE_{ATTACH,DETACH}_BACKING. It is
just that
the driver does not make use of them. That makes this less
of an
issue because the driver can be fixed anytime (finger
crossed that the
emulator won't have bugs in these untested paths). This
issue becomes
more urgent because we are considering adding a new HW
command[1]
where page moves will be disallowed. We definitely don't
want a HW
command that is inherently insecure, if BO pages pinned for
the
lifetime is considered a security issue on desktops.
Yeah, that's probably not such a good idea :)
Regards,
Christian.
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