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charset=UTF-8 Date: Wed, 19 Aug 2026 10:39:52 -0400 Message-Id: Cc: "Suren Baghdasaryan" , "Michal Hocko" , "Brendan Jackman" , "Johannes Weiner" , , , , To: "Qi Xi" , "Andrew Morton" , "Vlastimil Babka" From: "Zi Yan" Subject: Re: [PATCH] mm/page_isolation: fix UBSAN shift-out-of-bounds warning X-Mailer: aerc 0.21.0 References: <20260818112855.3831692-1-xiqi2@huawei.com> In-Reply-To: X-ClientProxiedBy: MN2PR05CA0038.namprd05.prod.outlook.com (2603:10b6:208:236::7) To IA0PR12MB8374.namprd12.prod.outlook.com (2603:10b6:208:40e::7) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: IA0PR12MB8374:EE_|DM6PR12MB4482:EE_ X-MS-Office365-Filtering-Correlation-Id: 44311ab9-b8b0-4e67-ec61-08defdffbb3c X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|7416014|376014|1800799024|366016|23010399003|18002099003|22082099003|56012099006|6133799003|11063799006|4143699003|10067099003; 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Once the lock is >>> dropped, it scans the MAX_ORDER_NR_PAGES-aligned window [start_pfn, >>> boundary_pfn) locklessly, only to skip the free pages already handled >>> above and to detect in-use pages straddling the boundary. Since this >>> scan only reads page state to decide how far to skip and returns -EBUSY >>> on a straddling in-use page, it does not take the lock. >>> >>> The window also covers the adjacent pageblock, whose free pages stay on >>> the normal movable/CMA freelist and can be allocated concurrently. So >>> after the scan observes PageBuddy(page), another CPU can allocate the >>> page, leaving a stale value in page->private that makes "1 << order" sh= ift >>> out of range. >>> >>> Use buddy_order_unsafe() to read the order exactly once (READ_ONCE), an= d >>> guard the shift with an order <=3D MAX_PAGE_ORDER check so it is never >>> performed with a bogus value. >>> >>> Fixes: b2c9e2fbba32 ("mm: make alloc_contig_range work at pageblock gra= nularity") >>> Signed-off-by: Qi Xi >>> --- >>> mm/page_isolation.c | 13 ++++++++----- >>> 1 file changed, 8 insertions(+), 5 deletions(-) >>> >> The fix makes sense to me. >> Please Cc stable for this. Thanks. >> >> Sashiko has two more concerns: >> >> 1. order can still be bogus even if it is within MAX_PAGE_ORDER. >> a. it can trigger VM_WARN_ON_ONCE(); >> b. it can skip arbitrary pages and miss an in-use compound page. >> >> For 1a, we can remove VM_WARN_ON_ONCE() and just fail >> isolate_single_pageblock() if PageBuddy crosses the boundary, since >> pageblock_isolate_and_move_free_pages() handles it. >> >> For 1b, after we makes the change in 1a, PageBuddy skip cannot land us >> beyond the boundary, so the concern is gone. >> >> >> 2. In PageCompound() branch, a bogus nr_pages can also cause the >> shift-out-of-bounds. >> >> It will need a different fix. >> >> >> Hi Qi, >> >> Do you mind fixing 1a along with this patch? >> >> For this patch alone, >> >> Reviewed-by: Zi Yan > Thanks for the review and suggestions.=C2=A0 I'll address them in a > follow-up: fold concern 1a into a v2, and send a separate patch > for concern 2.=C2=A0 Both will carry Cc: stable. Thanks. For concern 2, I used Claude to do some investigation and come up with the code below, feel free to use it as a reference. if (PageCompound(page)) { struct page *head =3D compound_head(page); unsigned long head_pfn =3D page_to_pfn(head); unsigned int order =3D compound_order(head); unsigned long nr_pages; /* compound_order() is racy. Cap it at MAX_FOLIO_ORDER. */ if (order > MAX_FOLIO_ORDER) goto failed; nr_pages =3D 1UL << order; /* * compound_head() is also racy, so the derived head_pfn needs * additional checks to make sure it is valid. Otherwise, just * fail the check. pfn comes from __first_valid_page() as a * legitmate PFN, so use it to check head_pfn. */ if (head_pfn > pfn || !IS_ALIGNED(head_pfn, nr_pages) || pfn - head_pfn >=3D nr_pages) goto failed; if (head_pfn + nr_pages <=3D boundary_pfn || PageHuge(head)) { pfn =3D head_pfn + nr_pages; continue; } /* * These pages are movable too, but they're * not expected to exceed pageblock_order. * * Let us know when they do, so we can add * proper free and split handling for them. */ VM_WARN_ON_ONCE_PAGE(PageLRU(page), page); VM_WARN_ON_ONCE_PAGE(page_has_movable_ops(page), page); goto failed; } --=20 Best Regards, Yan, Zi