Subject: Re: [PATCH v4 2/2] mm/gup/writeback: add callbacks for inaccessible pages

From: Dave Hansen <[email-masked]>
Date: Tue, 14 Apr 2020 11:50:16 -0700

On 4/14/20 9:03 AM, Claudio Imbrenda wrote:
> On Mon, 13 Apr 2020 13:22:24 -0700
> Dave Hansen <[email-masked]> wrote:
> 
>> On 3/6/20 5:25 AM, Claudio Imbrenda wrote:
>>> On s390x the function is not supposed to fail, so it is ok to use a
>>> WARN_ON on failure. If we ever need some more finegrained handling
>>> we can tackle this when we know the details.  
>>
>> Could you explain a bit why the function can't fail?
> 
> the concept of "making accessible" is only to make sure that accessing
> the page will not trigger faults or I/O or DMA errors. in general it
> does not mean freely accessing the content of the page in cleartext. 
> 
> on s390x, protected guest pages can be shared. the guest has to
> actively share its pages, and in that case those pages are both part of
> the protected VM and freely accessible by the host.

Oh, that's interesting.

It sounds like there are three separate concepts:
1. Protection
2. Sharing
3. Accessibility

Protected pages may be shared and the request of the guest.
Shared pages' plaintext can be accessed by the host.  For unshared
pages, the host can only see ciphertext.

I wonder if Documentation/virt/kvm/s390-pv.rst can be beefed up with
some of this information.  It seems a bit sparse on this topic.

As it stands, if I were modifying generic code, I don't think I'd have
even a chance of getting an arch_make_page_accessible() in the right spot.

> in our case "making the page accessible" means:
...
>  - if the page was not shared, first encrypt it and then make it
>    accessible to the host (both operations performed securely and
>    atomically by the hardware)

What happens to the guest's view of the page when this happens?  Does it
keep seeing plaintext?

> then the page can be swapped out, or used for direct I/O (obviously if
> you do I/O on a page that was not shared, you cannot expect good
> things to happen, since you basically corrupt the memory of the guest).

So why even allow access to the encrypted contents if the host can't do
anything useful with it?  Is there some reason for going to the trouble
of encrypting it and exposing it to the host?

> on s390x performing I/O directly on protected pages results in (in
> practice) unrecoverable I/O errors, so we want to avoid it at all costs.

This is understandable, but we usually steer I/O operations in places
like the DMA API, not in the core VM.

We *have* the concept of pages to which I/O can't be done.  There are
plenty of crippled devices where we have to bounce data into a low
buffer before it can go where we really want it to.  I think the AMD SEV
patches do this, for instance.

> accessing protected pages from the CPU triggers an exception that can
> be handled (and we do handle it, in fact)
> 
> now imagine a buggy or malicious qemu process crashing the whole machine
> just because it did I/O to/from a protected page. we clearly don't want
> that.

Is DMA disallowed to *all* protected pages?  Even pages which the guest
has explicitly shared with the host?

>>> @@ -2807,6 +2807,13 @@ int __test_set_page_writeback(struct page
>>> *page, bool keep_write) inc_zone_page_state(page,
>>> NR_ZONE_WRITE_PENDING); }
>>>  	unlock_page_memcg(page);
>>> +	access_ret = arch_make_page_accessible(page);
>>> +	/*
>>> +	 * If writeback has been triggered on a page that cannot
>>> be made
>>> +	 * accessible, it is too late to recover here.
>>> +	 */
>>> +	VM_BUG_ON_PAGE(access_ret != 0, page);
>>> +
>>>  	return ret;
>>>  
>>>  }  
>>
>> This seems like a really odd place to do this.  Writeback is specific
>> to block I/O.  I would have thought there were other kinds of devices
>> that matter, not just block devices.
> 
> well, yes and no. for writeback (block I/O and swap) this is the right
> place. at this point we know that the page is present and nobody else
> has started doing I/O yet, and I/O will happen soon-ish. so we make the
> page accessible. there is no turning back here, unlike pinning. we
> are not allowed to fail, we can't 

This description sounds really incomplete to me.

Not all swap involved device I/O.  For instance, zswap doesn't involve
any devices.  Would zswap need this hook?