Re: [RFC] /dev/ioasid uAPI proposal

[Jason Wang <jasowang@redhat.com>]

在 2021/6/8 上午3:41, Alex Williamson 写道:
> On Mon, 7 Jun 2021 16:08:02 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
>
>> On Mon, Jun 07, 2021 at 12:59:46PM -0600, Alex Williamson wrote:
>>
>>>> It is up to qemu if it wants to proceed or not. There is no issue with
>>>> allowing the use of no-snoop and blocking wbinvd, other than some
>>>> drivers may malfunction. If the user is certain they don't have
>>>> malfunctioning drivers then no issue to go ahead.
>>> A driver that knows how to use the device in a coherent way can
>>> certainly proceed, but I suspect that's not something we can ask of
>>> QEMU.  QEMU has no visibility to the in-use driver and sketchy ability
>>> to virtualize the no-snoop enable bit to prevent non-coherent DMA from
>>> the device.  There might be an experimental ("x-" prefixed) QEMU device
>>> option to allow user override, but QEMU should disallow the possibility
>>> of malfunctioning drivers by default.  If we have devices that probe as
>>> supporting no-snoop, but actually can't generate such traffic, we might
>>> need a quirk list somewhere.
>> Compatibility is important, but when I look in the kernel code I see
>> very few places that call wbinvd(). Basically all DRM for something
>> relavent to qemu.
>>
>> That tells me that the vast majority of PCI devices do not generate
>> no-snoop traffic.
> Unfortunately, even just looking at devices across a couple laptops
> most devices do support and have NoSnoop+ set by default.  I don't
> notice anything in the kernel that actually tries to set this enable (a
> handful that actively disable), so I assume it's done by the firmware.


I wonder whether or not it was done via ACPI:

"

6.2.17 _CCA (Cache Coherency Attribute) The _CCA object returns whether 
or not a bus-master device supports hardware managed cache coherency. 
Expected values are 0 to indicate it is not supported, and 1 to indicate 
that it is supported. All other values are reserved.

...

On Intel platforms, if the _CCA object is not supplied, the OSPM will 
assume the devices are hardware cache coherent.

"

Thanks


> It's not safe for QEMU to make an assumption that only GPUs will
> actually make use of it.
>
>>>> I think it makes the software design much simpler if the security
>>>> check is very simple. Possessing a suitable device in an ioasid fd
>>>> container is enough to flip on the feature and we don't need to track
>>>> changes from that point on. We don't need to revoke wbinvd if the
>>>> ioasid fd changes, for instance. Better to keep the kernel very simple
>>>> in this regard.
>>> You're suggesting that a user isn't forced to give up wbinvd emulation
>>> if they lose access to their device?
>> Sure, why do we need to be stricter? It is the same logic I gave
>> earlier, once an attacker process has access to wbinvd an attacker can
>> just keep its access indefinitely.
>>
>> The main use case for revokation assumes that qemu would be
>> compromised after a device is hot-unplugged and you want to block off
>> wbinvd. But I have a hard time seeing that as useful enough to justify
>> all the complicated code to do it...
> It's currently just a matter of the kvm-vfio device holding a reference
> to the group so that it cannot be used elsewhere so long as it's being
> used to elevate privileges on a given KVM instance.  If we conclude that
> access to a device with the right capability is required to gain a
> privilege, I don't really see how we can wave aside that the privilege
> isn't lost with the device.
>
>> For KVM qemu can turn on/off on hot plug events as it requires to give
>> VM security. It doesn't need to rely on the kernel to control this.
> Yes, QEMU can reject a hot-unplug event, but then QEMU retains the
> privilege that the device grants it.  Releasing the device and
> retaining the privileged gained by it seems wrong.  Thanks,
>
> Alex
>