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Wang , John Hubbard , "Huang, Ying" , Ryan Roberts , Alexander Potapenko , Kees Cook , linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1 Date: Thu, 05 Dec 2024 12:38:21 -0500 X-Mailer: MailMate (1.14r6065) Message-ID: <9812FEF4-73AB-4B83-88F8-EC21BE37147C@nvidia.com> In-Reply-To: <434596ce-5514-42f3-84f5-f8a70954f058@suse.cz> 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> <056a2305-db07-4168-868b-164493ff8587@redhat.com> <434596ce-5514-42f3-84f5-f8a70954f058@suse.cz> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable X-ClientProxiedBy: BL1PR13CA0111.namprd13.prod.outlook.com (2603:10b6:208:2b9::26) To DS7PR12MB9473.namprd12.prod.outlook.com (2603:10b6:8:252::5) MIME-Version: 1.0 X-MS-PublicTrafficType: Email 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2+NSBAfR3SRQStXoU9eP7mdkzLaQFo5YIEyZgtXYPWYii50bWIKo8zFouH/cJca0ywxqjGDp34DAumfW2zqLyxpT+E8F0JO6ybWQw6IJQTFR89QLT3D3t0hfZynsgvFFjfEEZDwKJ1N160Jg0h5LufveZnUozV93BJPnn1jeOjWs87W8tVZoCYsbwF9C3Ak/XOJM9kK/LOJq66vaA0qQ3cS73zj3c07IfLn/0L9aaGe/QWJwhgzPPexQNnHTlW09V2MknID2lvell1fpI8nIiSoYbow7ZLO3tCRelYhXs71OgQAgivcL6xfLntxay2gihC5t+BKnuBAnjl8OAnH7D1chAYULxGtqeQUzGDfhojgayx1M= 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 5 Dec 2024, at 12:24, Vlastimil Babka wrote: > On 12/5/24 17:05, Zi Yan wrote: >> On 5 Dec 2024, at 3:10, David Hildenbrand wrote: >>>> >>>> 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_DEFA= ULT_ON on systems to prevent exposing un-zeroed data to userspace, when i= t doesn't end up doing the flush we really need. >> >> Hi Geert, >> >> Is it possible to run a 32bit kernel with HIGHMEM and >> CONFIG_INIT_ON_ALLOC_DEFAULT_ON on the machine (of course with my patc= h >> reverted)? Just to check my reasoning below. >> >> Thanks. >> >> >> Yes, it should work, since I forgot the actual issue is HIGHMEM+cache = flush, not just cache flush is needed after clearing user page. >> >> For arch which needs to flush cache after clearing user page, with HIG= HMEM, >> init_on_alloc first clears the page using kmap_addr0 without flushing >> the cache, then clear_user_page() clears the page using kmap_addr1 >> with cache flush. After returning to userspace, the cache lines of >> kmap_addr0 will be evicted and written back to RAM eventually, corrupt= ing >> user data with 0s, because no one flushes them before returning to use= rspace. >> >> For a proper fix, I will add ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE and >> make mips, sh, sparc, arm, xtensa, nios2, m68k, parisc, csky, arc, and= powerpc >> select it, then make alloc_zeroed() returns false if >> ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE is enabled. >> >> If my reasoning above is verified to be true, I will send a separate p= atch >> to disable CONFIG_INIT_ON_ALLOC_DEFAULT_ON if HIGHMEM && >> ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE. > > If your reasoning is true, wouldn't any other user of kmap_local_page()= of a > highpage on such system also leave the cache unflushed in case the page= is > ever reused as a userspace page? If the page is written and no cache is flushed, yes. But if the page is r= ead and the cache lines are clean, no write back will be done. I wonder in what scenarios kernel writes to user pages, besides copy_to_u= ser*(), which should handle the cache flush. Best Regards, Yan, Zi