linux-kernel - Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap read fails

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Message-ID: <21003e7a-01e4-c751-dd41-fce4149d424c@redhat.com>
Date:   Tue, 19 Apr 2022 13:14:29 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org,
        willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, surenb@...gle.com, minchan@...nel.org,
        peterx@...hat.com, sfr@...b.auug.org.au, rcampbell@...dia.com,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap
 read fails

On 19.04.22 10:08, Alistair Popple wrote:
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 19.04.22 09:29, Miaohe Lin wrote:
>>> On 2022/4/19 11:51, Alistair Popple wrote:
>>>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>>>
>>>>> There is a bug in unuse_pte(): when swap page happens to be unreadable,
>>>>> page filled with random data is mapped into user address space. In case
>>>>> of error, a special swap entry indicating swap read fails is set to the
>>>>> page table. So the swapcache page can be freed and the user won't end up
>>>>> with a permanently mounted swap because a sector is bad. And if the page
>>>>> is accessed later, the user process will be killed so that corrupted data
>>>>> is never consumed. On the other hand, if the page is never accessed, the
>>>>> user won't even notice it.
>>>>
>>>> Hi Miaohe,
>>>>> It seems we're not actually using the pfn that gets stored in the special swap
>>>> entry here. Is my understanding correct? If so I think it would be better to use
>>>
>>> Yes, you're right. The pfn is not used now. What we need here is a special swap entry
>>> to do the right things. I think we can change to store some debugging information instead
>>> of pfn if needed in the future.
>>>
>>>> the new PTE markers Peter introduced[1] rather than adding another swap entry
>>>> type.
>>>
>>> IIUC, we should not reuse that swap entry here. From definition:
>>>
>>> PTE markers
>>> `========='
>>> ...
>>> PTE marker is a new type of swap entry that is ony applicable to file
>>> backed memories like shmem and hugetlbfs.  It's used to persist some
>>> pte-level information even if the original present ptes in pgtable are
>>> zapped.
>>>
>>> It's designed for file backed memories while swapin error entry is for anonymous
>>> memories. And there has some differences in processing. So it's not a good idea
>>> to reuse pte markers. Or am I miss something?
>>
>> I tend to agree. As raised in my other reply, maybe we can simply reuse
>> hwpoison entries and update the documentation of them accordingly.
> 
> Unless I've missed something I don't think PTE markers should be restricted
> solely to file backed memory. It's true that the only user of them at the moment
> is UFFD-WP for file backed memory, but PTE markers are just a special swap entry
> same as what is added here.

There is a difference.

What we want here is "there used to be something mapped but it's not
readable anymore. Please fail hard when userspace tries accessing
this.". Just like with hwpoison entries.

What a pte marker expresses is that "here is nothing mapped right now
but we have additional metadata available here. For file-backed memory,
it translates to: If we ever touch this page, lookup the pagecache what
to map here."

In the anonymous memory world, this would map to "populate the zeropage
or a fresh anonymous page on access." and keep the metadata around.

Yes, one might argue that for uffd-wp on anonymous memory it might make
sense to use pte marker as well, when no page has been populated yet.

IIRC, trying to arm uffd-wp when there is nothing populated yet will
simply get ignored.

> 
> That said I don't think there has been any attempt to make PTE markers work for
> anything other than UFFD-WP because it was unclear if there ever would be
> another user.

We discussed using it for softdirty tracking as well.

-- 
Thanks,

David / dhildenb