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[2003:d8:2f2d:5d00:f1a3:2f30:6575:9425]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a892e59659sm2258050f8f.77.2025.06.27.03.13.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 27 Jun 2025 03:13:49 -0700 (PDT) Message-ID: <609409c7-91a8-4898-ab29-fa00eb36df02@redhat.com> Date: Fri, 27 Jun 2025 12:13:48 +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 v2 1/1] mm/rmap: fix potential out-of-bounds page table access during batched unmap To: Barry Song <21cnbao@gmail.com>, Lance Yang Cc: akpm@linux-foundation.org, 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, lorenzo.stoakes@oracle.com, 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, Lance Yang References: <20250627062319.84936-1-lance.yang@linux.dev> <1d39b66e-4009-4143-a8fa-5d876bc1f7e7@linux.dev> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 27.06.25 09:36, Barry Song wrote: > On Fri, Jun 27, 2025 at 7:15 PM Lance Yang wrote: >> >> >> >> On 2025/6/27 14:55, Barry Song wrote: >>> On Fri, Jun 27, 2025 at 6:52 PM Barry Song <21cnbao@gmail.com> wrote: >>>> >>>> On Fri, Jun 27, 2025 at 6:23 PM Lance Yang wrote: >>>>> >>>>> From: Lance Yang >>>>> >>>>> As pointed out by David[1], the batched unmap logic in try_to_unmap_one() >>>>> can read past the end of a PTE table if a large folio is mapped starting at >>>>> the last entry of that table. It would be quite rare in practice, as >>>>> MADV_FREE typically splits the large folio ;) >>>>> >>>>> So let's fix the potential out-of-bounds read by refactoring the logic into >>>>> a new helper, folio_unmap_pte_batch(). >>>>> >>>>> The new helper now correctly calculates the safe number of pages to scan by >>>>> limiting the operation to the boundaries of the current VMA and the PTE >>>>> table. >>>>> >>>>> In addition, the "all-or-nothing" batching restriction is removed to >>>>> support partial batches. The reference counting is also cleaned up to use >>>>> folio_put_refs(). >>>>> >>>>> [1] https://lore.kernel.org/linux-mm/a694398c-9f03-4737-81b9-7e49c857fcbe@redhat.com >>>>> >>>> >>>> What about ? >>>> >>>> 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 spans across two PMDs, >>>> particularly after being remapped with mremap(). This patch fixes the >>>> potential out-of-bounds access by capping the batch at vm_end and the PMD >>>> boundary. >>>> >>>> It also refactors the logic into a new helper, folio_unmap_pte_batch(), >>>> which supports batching between 1 and folio_nr_pages. This improves code >>>> clarity. Note that such cases are rare in practice, as MADV_FREE typically >>>> splits large folios. >>> >>> Sorry, I meant that MADV_FREE typically splits large folios if the specified >>> range doesn't cover the entire folio. >> >> Hmm... I got it wrong as well :( It's the partial coverage that triggers >> the split. >> >> how about this revised version: >> >> 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 spans across two >> PMDs, particularly after being remapped with mremap(). This patch fixes >> the potential out-of-bounds access by capping the batch at vm_end and the >> PMD boundary. >> >> It also refactors the logic into a new helper, folio_unmap_pte_batch(), >> which supports batching between 1 and folio_nr_pages. This improves code >> clarity. Note that such boundary-straddling cases are rare in practice, as >> MADV_FREE will typically split a large folio if the advice range does not >> cover the entire folio. > > I assume the out-of-bounds access must be fixed, even though it is very > unlikely. It might occur after a large folio is marked with MADV_FREE and > then remapped to an unaligned address, potentially crossing two PTE tables. Right. If it can be triggered from userspace, it doesn't matter how likely/common/whatever it is. It must be fixed. > > A batch size between 2 and nr_pages - 1 is practically rare, as we typically > split large folios when MADV_FREE does not cover the entire folio range. > Cases where a batch of size 2 or nr_pages - 1 occurs may only happen if a > large folio is partially unmapped after being marked MADV_FREE, which is > quite an unusual pattern in userspace. I think the point is rather "Simplify the code by not special-casing for completely mapped folios, there is no real reason why we cannot batch ranges that don't cover the complete folio.". -- Cheers, David / dhildenb