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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=ny7xibCq; 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 33D33606A31C for ; Tue, 20 Oct 2026 12:59:34 +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=ny7xibCq Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D0F91F000C4; Tue, 20 Oct 2026 12:59:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1792501174; bh=T9RUmLmMu1KGYjGG2ltedL/onlV4Qx016VCDjCFQv/g=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=ny7xibCqSRC4KpUWk4NQfqIwK0iffFjwPWWzmNxQ1hrSEQRHhJTeBRNDhlOl9uoE2 /tThHfSCbz8U8niQDTCTdRaRCZPMRIkgNShXWTbS8ltb4u2W1RGIiRV9bd6Fr7NOI0 hZIU2F8u9khb7dIeu95F8gravV0Qbu8W4GFfSrh01mXDhXob5kCsHfDkJERkK2hJ+w c/IX6vHYuGShkCcnbUiE+ku5Ss0O+68FgbzxTTHuQ3CPHbsVylpxGoWaarHg34++7A dXpr6wj1L2jJ23hxEXgwj/M3RZUnCI9uVXBory1bQDQHsM2sALggSorc6I/ifM8Ntl FhPK/gnhGiiew== Message-ID: <58126141-6915-4abd-8e93-93c4aac474a8@kernel.org> Date: Tue, 20 Oct 2026 14:59:30 +0200 X-Mailing-List: linux-kernel@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: [BUG] mm/ksm: KSM merge drops userfaultfd RWP markers To: shaos , akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20261020110757.960025-1-shawnq1@163.com> 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 10/20/26 13:07, shaos wrote: > Hello, > > KSM can merge anonymous pages in a VMA registered for userfaultfd write-protect (RWP), then replace an armed PTE with an ordinary accessible PTE. The VMA remains registered, but a read of the replaced page completes without the expected UFFD_PAGEFAULT_FLAG_RWP event. > > The RWP PTE is represented by PAGE_NONE plus the architecture's _PAGE_UFFD marker. In replace_page(), KSM creates the replacement with mk_pte(kpage, vma->vm_page_prot) and does not preserve that state from orig_pte. The PoC observes pages_sharing increase, reads the replaced page without an event, then re-arms it and receives the RWP event again. A non-merging control keeps the marker and continues to fault. > > The same replacement path also loses soft-dirty state; that separate observable is documented at https://gist.github.com/shaos26/9db36768f01eb83c6aa14ee9fc057388. This report focuses on userfaultfd RWP tracking. > > HEAD commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907 > source version: Linux 7.3.0-rc7 > architecture: x86-64 > compiler: gcc 11.4.0, GNU ld 2.38 > QEMU: 6.2.0 > > console output: https://gist.githubusercontent.com/shaos26/e887665d7868dc323d73a1d678cacd3d/raw/e3e39212903e5bd5ba12f55a94ba7ae5b25bb238/console-output.txt > kernel config: https://gist.githubusercontent.com/shaos26/e887665d7868dc323d73a1d678cacd3d/raw/3cd943ba8c6026b43ef016a37d07efd2533a317a/kernel-config.txt > reproducer: https://gist.githubusercontent.com/shaos26/e887665d7868dc323d73a1d678cacd3d/raw/7fc8685bb0cc01fca488fc096d4859b8b70af3fc/poc.c > > Build the kernel from the revision above with the attached config. Build the static PoC with: > > gcc -static -O2 -pthread -Wall -Wextra -o poc poc.c > > The program is intended to run as /init in a disposable initramfs and mounts procfs and sysfs itself. Boot it with: > > qemu-system-x86_64 -m 1G -smp 2 -machine q35 -nographic -no-reboot \ > -kernel build/arch/x86/boot/bzImage -initrd initramfs.cpio.gz \ > -append "console=ttyS0 rdinit=/init panic=-1 oops=panic loglevel=6" > > Two fresh positive boots reported RESULT=KSM_RWP_MARKER_LOSS_CONFIRMED; the close non-merging control reported RESULT=KSM_CONTROL_TRAPS_PRESERVED. No privilege escalation, memory corruption, or cross-process access is claimed; the defect is loss of the caller's RWP access tracking. I checked the affected code against upstream master f47d223627f8; this source path appears unchanged there, but I have only reproduced the behavior on v7.3-rc7. > > Thanks, > > shaos > Always a good idea to look at the list before posting stuff. https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/ -- Cheers, David