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

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Message-ID: <b2142993-e77f-4998-ba87-0d51629c0f6d@suse.cz>
Date: Wed, 4 Dec 2024 18:46:36 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Zi Yan <ziy@...dia.com>, Matthew Wilcox <willy@...radead.org>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>,
 David Hildenbrand <david@...hat.com>, 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 12/4/24 18:33, 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?

Maybe the arch also defines some hooks like arch_kmap_local_post_unmap() ?

As for the fix, could it rely on e.g. __HAVE_ARCH_COPY_USER_HIGHPAGE instead
of CONFIG_MIPS? That affects more arches, I don't know if we broke only mips
or others too.

> Best Regards,
> Yan, Zi