From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794597732; b=HkKB78wG7LvKoboR1Q6uwW9a3+V3/3PqtmsrRWckFSDFnYf4XBggsLuRJrKs0JIHqd9q Ey48oV9frA0KJcartBID57JHFisDeBRDYdNyhPg9zxw3cchkAZpnLKiElNXAZHHRyaBw+ qTaZhCRiH1p+azzsmJk+cPi6Jr8lYunl+RBSNgj074moqkKuiWi6QoLixXwauz1cmD4iQ 0OzTCjUvRFDb2nlZhhbtXSUxZNpEaG36vOVwb/CLgQHBMgG12ZoqoU7WK6TCsW7XuBFdk 3tupPtMQBmp+2fSSN0EZtzY0jsd1tWp2m4mooTA+qMdljqUXa8cd2wVhdlgAuQ2eAwg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794597732; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=Fdl4VOJT6zViP7jtfnU1Xg3aoOGQL41O0hxbPwDlR+A=; b=nIyV2fQezND2RRdbuMaUvbtTZiCj+TlgBR/g877ffslgzdabz5gIbn3Z8J8GBTJ1j8z5 Vb8zAzi2cWHYYBdMqdDAVzUkcKo8+ah2Lel83/gCMjDffRxyBKUzWU0i7QKThpC25lwR2 AlIVCd5Avi8TWBJxRW0uwv/yPXbpA10YmMNhKJavaGllyxq0zsF4dMcm2nbehHF5G2sEL SRCMHDiRb1rchL13uzr9HwaS/WE3PcyCikjsxzxAFQkh87KlzToxfOJxPh5AAzGq0L9DX vPWcSfBOBmH8qS47EULGndAK9iWV6B76Doa1RPDQl73DYPG5e5ImeFTjsMFjKibolLg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=NX6l4vlx; 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 81AF3606A890; Fri, 13 Nov 2026 19:22:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=NX6l4vlx Received: by smtp.kernel.org (Postfix) with ESMTPSA id F10861F000C8; Fri, 13 Nov 2026 19:22:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794597732; bh=Fdl4VOJT6zViP7jtfnU1Xg3aoOGQL41O0hxbPwDlR+A=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=NX6l4vlxAsFOjaj+ZundCIjebkwjdIPIUBB6r6xto4qN1LhAmxdOJZlzrDEKEFlHX HE+RzVuSSPFuI5p/wzPEd1EWoL48luhiQrH5Fuylbug07Ef3zaiXlkH2raN+byKwcx ETtjAefB4pa1W94nXtaIlrAAzMDf7b2jtwqGkoqUv3j2kS3nzrIYUPM0FCJ6ZQ/TUH Pg9l/Ahp2pnUhstVIekqNTSYv5F0iZAVv+3d7lVWPZopB2q+jIlDKPVOg625y+x55w 2HbExloMEtf2bl5fPgILQoRc23yrhUKrH3yaVEEApNzX0fSs170YpwQdkw5kr35VOZ TavaBPQLZKR1g== Date: Fri, 13 Nov 2026 19:22:06 +0000 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , "Liam R. Howlett" , Vlastimil Babka , Jann Horn , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, Tal Zussman , stable@vger.kernel.org Subject: Re: [PATCH] mm/madvise: disallow guard region installation in uffd-armed VMAs Message-ID: References: <20261112-disallow-uffd-guard-install-v1-1-bcf8b1aaa72e@kernel.org> <0e3e876e-c3df-4660-ad89-a5f0fd7dedc8@kernel.org> <0d6e29f8-54fc-427a-a979-1a8c6f4e3f06@kernel.org> X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <0d6e29f8-54fc-427a-a979-1a8c6f4e3f06@kernel.org> On Fri, Nov 13, 2026 at 08:15:34PM +0100, David Hildenbrand (Arm) wrote: > On 11/13/26 14:59, Lorenzo Stoakes (ARM) wrote: > > On Fri, Nov 13, 2026 at 12:59:00AM +0100, David Hildenbrand (Arm) wrote: > >> On 11/12/26 18:38, Lorenzo Stoakes (ARM) wrote: > >>> In order to establish guard regions, guard markers are installed in the > >>> page tables of a mapped VMA. > >>> > >>> However, problems arise when one tries to combine this with registering a > >>> VMA for userfaultfd, as such operations may themselves need to install page > >>> table markers such as uffd-wp. > >> > >> How does this interact with UFFDIO_POISON markers that can outlive even > >> userfaultfd registration? > > > > The zap clears poison markers, as is standard. It's really horrible this logic. > > > > So the code path for zap used by the guard install (via zap_vma_range()) is: > > > > zap_vma_range() > > -> zap_vma_range_batched() with NULL zap_details > > -> __zap_vma_range() > > -> zap_p4d_range() > > ... > > -> zap_pte_range() > > -> do_zap_pte_range() > > -> zap_nonpresent_ptes() > > > > And zap_details is NULL. > > > In zap_non_present_ptes(): > > > > ... > > } else if (softleaf_is_uffd_wp_marker(entry)) { > > if (... !zap_drop_markers(details)) > > return 1; > > } else if (softleaf_is_guard_marker(entry)) { > > ... > > if (!zap_drop_markers(details)) > > return 1; > > } else if (softleaf_is_hwpoison(entry) || > > softleaf_is_poison_marker(entry)) { > > if (!should_zap_cows(details)) > > return 1; > > } ... > > > > And zap_drop_markers() checks zap_details && zap_details->flags & > > ZAP_FLAG_DROP_MARKER. > > Right, my intuition tells me to just nuke that thing by setting > ZAP_FLAG_DROP_MARKER. > > MADV_GUARD, is just like munmap() another form of "zap it all and protect it". > >>> This is especially problematic because guard region installation causes the > >>> regions to be zapped first, meaning that the removal of userfaultfd-tracked > >>> ranges will not be logged in this case. > >> > >> Again, similar to trying to uffd-wp-protect'ing a hwpoison marker (which will be > >> ignored), and then zapping that. > >> > >> Isn't guard marker installation/removal just another form of zapping? > > > > I mean users being weird could decide to use it for that rather than > > MADV_DONTNEED, but zapping is a side effect really. > > > > Really the idea is that you might have users who, for whatever reason, > > faulted in some range, and now want to make that a guard range, so for > > convenience we zap it for them :) > > I think not just convenience. There was this problem with faultaround / THP > placement IIRC ... > > I'd just zap everything that's there. That's literally what the user asked for. > > > > >> > >> So to me this all sounds like "if user space does something stupid, it's user > >> space's fault". > > > > Unfortunately, as per Tal's series, there's more to it than that (linked in > > Closes: tags). > > > > You can get an infinite syscall restart (ugh) if you do this on a shmem > > range (Tal's 9/12) so it's a kernel bug that _has_ to be fixed. > > > > The other bits are just about a user losing track of uffd stuff if they > > guard/unguard, as you say. > > > > But every other zap code path does the uffd remove event stuff, so this is > > still an exception. > > > > If you support uffd, you have to add a whole bunch of horrifying, awful, > > terrible open-coded uffd crap, e.g. the userfaultfd_remove() etc. And you > > have to drop the bloody lock and reacquire it too and make sure all that > > works correctly (...!) > > > > Anyway there's basically 3 options here: > > > > 1. do something like what Tal's series does - add a TON of complexity for a > > use case that nobody uses to get behaviour on something nobody sane > > should care about (i.e. uffd/guard/unguard/expect uffd correctness). > > > > 2. Zap with ZAP_FLAG_DROP_MARKER, and silently lose uffd-wp state but > > 'support' uffd. It does fix the 9/12 bug but then subtly breaks uffd. > > Right, that's what I would do. > > MADV_GUARD_INSTALL zaps everything. > > It already zaps swap entries, migration entries, hwpoison entries ... so just > zap everything. > > [...] > > > > > If you're really concerned though, we could go with option 2, which will > > just make UFFD behave differently for guard regions. > > > > Not really concerned, it's just the most consistent one for me. Zap everything. > The user asks for it, like he would ask with munmap(). > > > (I regret, obviously, ever allowing this for uffd ranges. And we _all_ > > regret that uffd was _ever_ allowed to be merged in this state...) > Yeah ... Ack will send a v2 doing that then :) Heads up that (naturally) the subject line will differ! -- Cheers, Lorenzo