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t=1793371189; bh=oKRevRmUwUIKRTw8Grw6ZvIs0InwSJnYXmbgD1Qkg6c=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=QlrsuYA6mex7jjvoZN82HmAjIvobJ74fu3R5Y8i+QFMLcTfyIePVM7xN1dyaZmXL7 GjfsXTrC2E34yDNgbOosBLx8P1/GWkFdsQisfKlchJofAycGG2qTtm4EUPhnyJFIOI JBk9cXZ8V6+rN2p4bMymv58olvcDnQUINMSfh8zSONnumclsw57PNhgNGT04t0Hrcg SBCkVAQskvK73TvZvea/JgHI4FLvM28tAO5Wi0s1EsMv4sL5sTmSK5ZUfy1PvOI5x2 5RpbnlG7PUdvXlVAxc7dECd1qmUxZp3CahSsHL4TIMHfFgNcTZ6cxwUXleTIORKHQk HPocaXD+1VfSw== Message-ID: <2c7aa5c1-3af3-4654-b244-1257d05c9d45@kernel.org> Date: Fri, 30 Oct 2026 15:39:40 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 0/2] mm/memfd: add F_SEAL_PUNCH_HOLE To: Jason Gunthorpe Cc: Samiullah Khawaja , Andrew Morton , Hugh Dickins , Baolin Wang , Muchun Song , Oscar Salvador , Jeff Layton , Chuck Lever , Alexander Aring , Shuah Khan , Gerd Hoffmann , Vivek Kasireddy , Jeff Xu , David Matlack , Pasha Tatashin , linux-mm@kvack.org, linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org References: <20261026220546.3114241-1-skhawaja@google.com> <20261030121113.GB3889730@ziepe.ca> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 9ud13exhcqrqgen9zc61f9b51uzfdo7j X-Spam-Status: No, score=-7.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 87A1B40003 X-Rspam-User: X-HE-Tag: 1793371190-408551 X-HE-Meta: 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 mv3bYD+I rvVJoZBC8V66RwjVfslUb8UN4MYtjno9SklwTJX7Cyz2XqFUOG2dO5Cm5u4vzZEFfo3BG4QnVXyZF1fhO+hYJnEggBx2TsAuV+QjI/8W+XffrsZPraObrBd7Ui8jJyQ6jddhATS8rpv1nh6g9Q9xDeBoDpy2r459lsoQhnPncPxqD/gFtUnQtCzmQ8Tb49OgbMvnyencu+aInxtO3sAnrxOc8yiNFEm+kZUvNFXp2V5ZIiMn0EP1VhafC0VWi0XgoC7BfoI5lLnnki/uAsb2fs3jGT/aAReg2XgeYx679H63rOI/FbNU6GIWqPt0vaR18AmoD2eXIA3SWHtbbA6IAuSW+K3Q0JY9hGvjVGfvJIJnMG8zs4p+UsXcSkZlOb7gMoQEB Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/30/26 13:11, Jason Gunthorpe wrote: > On Fri, Oct 30, 2026 at 11:20:30AM +0100, David Hildenbrand (Arm) wrote: >> On 10/26/26 23:05, Samiullah Khawaja wrote: >>> This series introduces F_SEAL_PUNCH_HOLE, a new memfd seal that prevents >>> holes from being punched into a shmem or hugetlbfs memfd while leaving >>> the file fully writable. >>> >>> Background >>> ========== >>> >>> Drivers such as udmabuf and iommufd (via IOMMU_IOAS_MAP_FILE) use >>> memfd_pin_folios() to pin the folios backing a memfd and hand them to a >>> device for DMA. Pinning keeps those folios allocated, but it does not >>> keep them in the file. If userspace later calls >>> fallocate(FALLOC_FL_PUNCH_HOLE) or madvise(MADV_REMOVE) on the range, >>> the folios are dropped from the page cache while the device continues >>> to use them. The pin holder is never notified, and from that point on >>> the device and the file silently diverge. >> >> We have the exact same situation when userspace maps anonymous folios into vfio >> and then desides to MADV_DONTNEED the pages. Or any other memory backend that is >> used for such purposes. >> >> User space must not zap/punch things that might be pinned elsewhere. >> >> That's not specific to memfd, udmabuf, iommufd or anything else. >> >> I don't see a reason to make userspace (that should not shoot itself into the >> foot) a kernel feature it must activate itself to not shoot itself into the foot. > > The issue is live update doesn't preserve those background punched > folios and now we have created an UAF attack on the kernel across > kexec if you don't do something about it. > > There was some notion here that with the right sealing flags we could > tell if everything was 'live update compatible' and then know that > these attacks are impossible. Just remember that there was any hole punch and refuse to live update this instance. We could likely implement "we saw hole punch on a pinned folio" as well, but I don't think that's required initially. No need for more shaky seals. -- Cheers, David