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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Subject: Re: [RFCv1 7/7] KVM: unmap guest memory using poisoned pages
Message-ID: <5ef83789-ffa5-debd-9ea2-50d831262237@redhat.com>
Date:   Fri, 9 Apr 2021 15:50:42 +0200
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>> It looks quite hacky (well, what did I expect from an RFC :) ) you can no
>> longer distinguish actually poisoned pages from "temporarily poisoned"
>> pages. FOLL_ALLOW_POISONED sounds especially nasty and dangerous -  "I want
>> to read/write a poisoned page, trust me, I know what I am doing".
>>
>> Storing the state for each individual page initially sounded like the right
>> thing to do, but I wonder if we couldn't handle this on a per-VMA level. You
>> can just remember the handful of shared ranges internally like you do right
>> now AFAIU.
> 
> per-VMA would not fly for file-backed (e.g. tmpfs) memory. We may need to
> combine PG_hwpoison with VMA flag. Maybe per-inode tracking would also be
> required. Or per-memslot. I donno. Need more experiments.

Indeed.

> 
> Note, I use PG_hwpoison now, but if we find a show-stopper issue where we
> would see confusion with a real poison, we can switch to new flags and
> a new swap_type(). I have not seen a reason yet.

I think we'll want a dedicate mechanism to cleanly mark pages as 
"protected". Finding a page flag you can use will be the problematic 
part, but should not be impossible if we have a good reason to do so 
(even if it means making the feature mutually exclusive with other 
features).

> 
>>  From what I get, you want a way to
>>
>> 1. Unmap pages from the user space page tables.
> 
> Plain unmap would not work for some use-cases. Some CSPs want to
> preallocate memory in a specific way. It's a way to provide a fine-grained
> NUMA policy.
> 
> The existing mapping has to be converted.
> 
>> 2. Disallow re-faulting of the protected pages into the page tables. On user
>> space access, you want to deliver some signal (e.g., SIGBUS).
> 
> Note that userspace mapping is the only source of pfn's for VM's shadow
> mapping. The fault should be allow, but lead to non-present PTE that still
> encodes pfn.

Makes sense, but I guess that's the part still to be implemented (see 
next comment).

> 
>> 3. Allow selected users to still grab the pages (esp. KVM to fault them into
>> the page tables).
> 
> As long as fault leads to non-present PTEs we are fine. Usespace still may
> want to mlock() some of guest memory. There's no reason to prevent this.

I'm curious, even get_user_pages() will lead to a present PTE as is, no? 
So that will need modifications I assume. (although I think it 
fundamentally differs to the way get_user_pages() works - trigger a 
fault first, then lookup the PTE in the page tables).

>> 4. Allow access to currently shared specific pages from user space.
>>
>> Right now, you achieve
>>
>> 1. Via try_to_unmap()
>> 2. TestSetPageHWPoison
>> 3. TBD (e.g., FOLL_ALLOW_POISONED)
>> 4. ClearPageHWPoison()
>>
>>
>> If we could bounce all writes to shared pages through the kernel, things
>> could end up a little easier. Some very rough idea:
>>
>> We could let user space setup VM memory as
>> mprotect(PROT_READ) (+ PROT_KERNEL_WRITE?), and after activating protected
>> memory (I assume via a KVM ioctl), make sure the VMAs cannot be set to
>> PROT_WRITE anymore. This would already properly unmap and deliver a SIGSEGV
>> when trying to write from user space.
>>
>> You could then still access the pages, e.g., via FOLL_FORCE or a new fancy
>> flag that allows to write with VM_MAYWRITE|VM_DENYUSERWRITE. This would
>> allow an ioctl to write page content and to map the pages into NPTs.
>>
>> As an extension, we could think about (re?)mapping some shared pages
>> read|write. The question is how to synchronize with user space.
>>
>> I have no idea how expensive would be bouncing writes (and reads?) through
>> the kernel. Did you ever experiment with that/evaluate that?
> 
> It's going to be double bounce buffer: on the guest we force swiotlb to
> make it go through shared region. I don't think it's a good idea.

So if it's already slow, do we really care? ;)

> 
> There are a number of way to share a memory. It's going to be decided by
> the way we get these pages unmapped in the first place.

I agree that shared memory can be somewhat problematic and would require 
tracking it per page.

-- 
Thanks,

David / dhildenb