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smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QlrsuYA6; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1086D606A31C; Fri, 30 Oct 2026 14:39:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=QlrsuYA6 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9B9CF1F000C1; Fri, 30 Oct 2026 14:39:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; 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-Mailing-List: linux-api@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list 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 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