Date:   Thu, 3 Dec 2020 17:01:32 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>
Cc:     Justin He <Justin.He@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Cornelia Huck <cohuck@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] vfio iommu type1: Bypass the vma permission check in
 vfio_pin_pages_remote()

On 03.12.20 16:43, Peter Xu wrote:
> On Thu, Dec 03, 2020 at 11:20:02AM +0000, Stefan Hajnoczi wrote:
>> On Wed, Dec 02, 2020 at 10:45:11AM -0500, Peter Xu wrote:
>>> On Wed, Dec 02, 2020 at 02:33:56PM +0000, Stefan Hajnoczi wrote:
>>>> On Wed, Nov 25, 2020 at 10:57:11AM -0500, Peter Xu wrote:
>>>>> On Wed, Nov 25, 2020 at 01:05:25AM +0000, Justin He wrote:
>>>>>>> I'd appreciate if you could explain why vfio needs to dma map some
>>>>>>> PROT_NONE
>>>>>>
>>>>>> Virtiofs will map a PROT_NONE cache window region firstly, then remap the sub
>>>>>> region of that cache window with read or write permission. I guess this might
>>>>>> be an security concern. Just CC virtiofs expert Stefan to answer it more accurately.
>>>>>
>>>>> Yep.  Since my previous sentence was cut off, I'll rephrase: I was thinking
>>>>> whether qemu can do vfio maps only until it remaps the PROT_NONE regions into
>>>>> PROT_READ|PROT_WRITE ones, rather than trying to map dma pages upon PROT_NONE.
>>>>
>>>> Userspace processes sometimes use PROT_NONE to reserve virtual address
>>>> space. That way future mmap(NULL, ...) calls will not accidentally
>>>> allocate an address from the reserved range.
>>>>
>>>> virtio-fs needs to do this because the DAX window mappings change at
>>>> runtime. Initially the entire DAX window is just reserved using
>>>> PROT_NONE. When it's time to mmap a portion of a file into the DAX
>>>> window an mmap(fixed_addr, ...) call will be made.
>>>
>>> Yes I can understand the rational on why the region is reserved.  However IMHO
>>> the real question is why such reservation behavior should affect qemu memory
>>> layout, and even further to VFIO mappings.
>>>
>>> Note that PROT_NONE should likely mean that there's no backing page at all in
>>> this case.  Since vfio will pin all the pages before mapping the DMAs, it also
>>> means that it's at least inefficient, because when we try to map all the
>>> PROT_NONE pages we'll try to fault in every single page of it, even if they may
>>> not ever be used.
>>>
>>> So I still think this patch is not doing the right thing.  Instead we should
>>> somehow teach qemu that the virtiofs memory region should only be the size of
>>> enabled regions (with PROT_READ|PROT_WRITE), rather than the whole reserved
>>> PROT_NONE region.
>>
>> virtio-fs was not implemented with IOMMUs in mind. The idea is just to
>> install a kvm.ko memory region that exposes the DAX window.
>>
>> Perhaps we need to treat the DAX window like an IOMMU? That way the
>> virtio-fs code can send map/unmap notifications and hw/vfio/ can
>> propagate them to the host kernel.
> 
> Sounds right.  One more thing to mention is that we may need to avoid tearing
> down the whole old DMA region when resizing the PROT_READ|PROT_WRITE region
> into e.g. a bigger one to cover some of the previusly PROT_NONE part, as long
> as if the before-resizing region is still possible to be accessed from any
> hardware.  It smells like something David is working with virtio-mem, not sure
> whether there's any common infrastructure that could be shared.

"somehow teach qemu that the virtiofs memory region should only be the
size of enabled regions" - for virtio-mem, I'm working on resizeable RAM
blocks/RAM memory regions. Fairly complicated, that's why I deferred
upstreaming it and still need to implement plenty of special cases for
atomic resizes (e.g., vhost-user).

But it's only one part of the puzzle for virtio-fs. AFAIU, it's not only
about resizing the region for virtio-fs - we can have PROT_NONE holes
anywhere inside there.

In vfio, you cannot shrink mappings atomically. Growing works, but
requires additional mappings (-> bad). So assume you mapped a file with
size X and want to resize it. You first have to unmap + remap with the
new size. This is not atomic, thus problematic.

For virtio-mem, we have a fix block size and can map/unmap in that
granularity whenever we populate/discard memory within the
device-managed region. I don't think that applies for files in case of
virtio-fs.


The real question is: do we even *need* DMA from vfio devices to
virtio-fs regions? If not (do guests rely on it? what does the spec
state?), just don't care about vfio at all and don't map anything.

But maybe I am missing something important

Q1: Is the virtio-fs region mapped into system address space, so we have
to map everything without a vIOMMU?

Q2: Is DMA from vfio devices to virtio-fs regions a valid use case?

-- 
Thanks,

David / dhildenb

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