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[2003:d8:2f40:b300:53f7:d260:aff4:7256]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538a3a5b7fsm138954025e9.10.2025.06.30.06.41.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 30 Jun 2025 06:41:23 -0700 (PDT) Message-ID: <498648fd-655e-47b3-8b7b-9c2ee11acc9b@redhat.com> Date: Mon, 30 Jun 2025 15:41:22 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap To: Lorenzo Stoakes , Lance Yang Cc: akpm@linux-foundation.org, 21cnbao@gmail.com, baolin.wang@linux.alibaba.com, chrisl@kernel.org, kasong@tencent.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-riscv@lists.infradead.org, ryan.roberts@arm.com, v-songbaohua@oppo.com, x86@kernel.org, huang.ying.caritas@gmail.com, zhengtangquan@oppo.com, riel@surriel.com, Liam.Howlett@oracle.com, vbabka@suse.cz, harry.yoo@oracle.com, mingzhe.yang@ly.com, stable@vger.kernel.org, Barry Song , Lance Yang References: <20250630011305.23754-1-lance.yang@linux.dev> <41483c78-84f2-42fc-b9ab-09823eb796c4@lucifer.local> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 30.06.25 15:39, Lorenzo Stoakes wrote: > On Mon, Jun 30, 2025 at 09:13:05AM +0800, Lance Yang wrote: >> From: Lance Yang >> >> As pointed out by David[1], the batched unmap logic in try_to_unmap_one() >> may read past the end of a PTE table when a large folio's PTE mappings >> are not fully contained within a single page table. >> >> While this scenario might be rare, an issue triggerable from userspace must >> be fixed regardless of its likelihood. This patch fixes the out-of-bounds >> access by refactoring the logic into a new helper, folio_unmap_pte_batch(). >> >> The new helper correctly calculates the safe batch size by capping the scan >> at both the VMA and PMD boundaries. To simplify the code, it also supports >> partial batching (i.e., any number of pages from 1 up to the calculated >> safe maximum), as there is no strong reason to special-case for fully >> mapped folios. >> >> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com >> >> Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") >> Cc: >> Acked-by: Barry Song >> Suggested-by: David Hildenbrand >> Suggested-by: Barry Song >> Signed-off-by: Lance Yang > > This LGTM: > > Reviewed-by: Lorenzo Stoakes > >> --- >> v2 -> v3: >> - Tweak changelog (per Barry and David) >> - Pick AB from Barry - thanks! >> - https://lore.kernel.org/linux-mm/20250627062319.84936-1-lance.yang@linux.dev >> >> v1 -> v2: >> - Update subject and changelog (per Barry) >> - https://lore.kernel.org/linux-mm/20250627025214.30887-1-lance.yang@linux.dev >> >> mm/rmap.c | 46 ++++++++++++++++++++++++++++------------------ >> 1 file changed, 28 insertions(+), 18 deletions(-) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index fb63d9256f09..1320b88fab74 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -1845,23 +1845,32 @@ void folio_remove_rmap_pud(struct folio *folio, struct page *page, >> #endif >> } >> >> -/* We support batch unmapping of PTEs for lazyfree large folios */ >> -static inline bool can_batch_unmap_folio_ptes(unsigned long addr, >> - struct folio *folio, pte_t *ptep) >> +static inline unsigned int folio_unmap_pte_batch(struct folio *folio, >> + struct page_vma_mapped_walk *pvmw, >> + enum ttu_flags flags, pte_t pte) >> { >> const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY; >> - int max_nr = folio_nr_pages(folio); >> - pte_t pte = ptep_get(ptep); >> + unsigned long end_addr, addr = pvmw->address; >> + struct vm_area_struct *vma = pvmw->vma; >> + unsigned int max_nr; >> + >> + if (flags & TTU_HWPOISON) >> + return 1; >> + if (!folio_test_large(folio)) >> + return 1; >> >> + /* We may only batch within a single VMA and a single page table. */ >> + end_addr = pmd_addr_end(addr, vma->vm_end); >> + max_nr = (end_addr - addr) >> PAGE_SHIFT; >> + >> + /* We only support lazyfree batching for now ... */ >> if (!folio_test_anon(folio) || folio_test_swapbacked(folio)) >> - return false; >> + return 1; >> if (pte_unused(pte)) >> - return false; >> - if (pte_pfn(pte) != folio_pfn(folio)) >> - return false; >> + return 1; >> >> - return folio_pte_batch(folio, addr, ptep, pte, max_nr, fpb_flags, NULL, >> - NULL, NULL) == max_nr; >> + return folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr, fpb_flags, >> + NULL, NULL, NULL); > > I guess this will conflict with David's changes, but maybe in this simpler case > and given this was existing code a bit less? Anyway let's see. It's a fix, so this is fine to go in first. (I already rebased on top of mm/mm-new where this is in) Acked-by: David Hildenbrand -- Cheers, David / dhildenb