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[2003:cb:c70f:7e00:75dc:9c40:33fa:b5f8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-434d52c0cfdsm53188135e9.33.2024.12.05.00.10.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 05 Dec 2024 00:10:24 -0800 (PST) Message-ID: <056a2305-db07-4168-868b-164493ff8587@redhat.com> Date: Thu, 5 Dec 2024 09:10:22 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1 To: Geert Uytterhoeven , Zi Yan Cc: Matthew Wilcox , Vlastimil Babka , linux-mm@kvack.org, Andrew Morton , Miaohe Lin , Kefeng Wang , John Hubbard , "Huang, Ying" , Ryan Roberts , Alexander Potapenko , Kees Cook , linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org References: <20241011150304.709590-1-ziy@nvidia.com> <9942C08D-C188-461C-B731-F08DE294CD2B@nvidia.com> <09B2AB6A-B122-4287-B97E-F800E511097E@nvidia.com> <995E365D-5B83-41B3-A46C-6493D203A761@nvidia.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Stat-Signature: 3jhndq8n3ahox7crjzt33wtgzmep185q X-Rspam-User: X-Rspamd-Queue-Id: 7234C1A000E X-Rspamd-Server: rspam08 X-HE-Tag: 1733386214-91689 X-HE-Meta: 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 yIg3rSIr 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 05.12.24 09:04, Geert Uytterhoeven wrote: > Hi Zi, > > On Wed, Dec 4, 2024 at 7:30 PM Zi Yan 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