linux-kernel - Re: [PATCH] mm: avoid zeroing user movable page twice with init_on

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Message-ID: <056a2305-db07-4168-868b-164493ff8587@redhat.com>
Date: Thu, 5 Dec 2024 09:10:22 +0100
From: David Hildenbrand <david@...hat.com>
To: Geert Uytterhoeven <geert@...ux-m68k.org>, Zi Yan <ziy@...dia.com>
Cc: Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
 linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
 Miaohe Lin <linmiaohe@...wei.com>, Kefeng Wang <wangkefeng.wang@...wei.com>,
 John Hubbard <jhubbard@...dia.com>, "Huang, Ying" <ying.huang@...el.com>,
 Ryan Roberts <ryan.roberts@....com>, Alexander Potapenko
 <glider@...gle.com>, Kees Cook <keescook@...omium.org>,
 linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org
Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with
 init_on_alloc=1

On 05.12.24 09:04, Geert Uytterhoeven wrote:
> Hi Zi,
> 
> On Wed, Dec 4, 2024 at 7:30 PM Zi Yan <ziy@...dia.com> wrote:
>> On 4 Dec 2024, at 12:33, Zi Yan wrote:
>>> On 4 Dec 2024, at 11:29, Matthew Wilcox wrote:
>>>> On Wed, Dec 04, 2024 at 11:16:51AM -0500, Zi Yan wrote:
>>>>>> So maybe the clearing done as part of page allocator isn't enough here.
>>>>>>
>>>>> Basically, mips needs to flush data cache if kmap address is aliased to
>>>>
>>>> People use "aliased" in contronym ways.  Do you mean "has a
>>>> non-congruent alias" or "has a congruent alias"?
>>>>
>>>>> userspace address. This means when mips has THP on, the patch below
>>>>> is not enough to fix the issue.
>>>>>
>>>>> In post_alloc_hook(), it does not make sense to pass userspace address
>>>>> in to determine whether to flush dcache or not.
>>>>>
>>>>> One way to fix it is to add something like arch_userpage_post_alloc()
>>>>> to flush dcache if kmap address is aliased to userspace address.
>>>>> But my questions are that
>>>>> 1) if kmap address will always be the same for two separate kmap_local() calls,
>>>>
>>>> No.  It just takes the next address in the stack.
>>>
>>> Hmm, if kmap_local() gives different addresses, wouldn’t init_on_alloc be
>>> causing issues before my patch? In the page allocator, the page is zeroed
>>> from one kmap address without flush, then clear_user_highpage() clears
>>> it again with another kmap address with flush. After returning to userspace,
>>> the user application works on the page but when the cache line used by
>>> init_on_alloc is written back (with 0s) at eviction, user data is corrupted.
>>> Am I missing anything? Or all arch with cache aliasing never enables
>>> init_on_alloc?
>>
>> Hi Geert,
>>
>> Regarding the above concern, have you ever had CONFIG_INIT_ON_ALLOC_DEFAULT_ON
>> for your MIPS machine and encountered any issue? Or let me know if my reasoning
>> above is flawed.
>>
>> To test it, I wonder if you can 1) revert my patch and 2) turn on
>> CONFIG_INIT_ON_ALLOC_DEFAULT_ON for your MIPS machine and run some applications
>> to see if any error happens.
> 
> That seems to work fine...
> 
> Kernel log confirms it's enabled:
> -mem auto-init: stack:off, heap alloc:off, heap free:off
> +mem auto-init: stack:off, heap alloc:on, heap free:off

If I'm not wrong that's expected ... because we'll be double-zeroing 
that memory, clearing the cache :)

I guess the question is, how *effective* is 
CONFIG_INIT_ON_ALLOC_DEFAULT_ON on systems to prevent exposing un-zeroed 
data to userspace, when it doesn't end up doing the flush we really need.

-- 
Cheers,

David / dhildenb