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=1794604190; b=RzT9gmsNiOJBgcLCvbRAYxD1Cu5dOFw+vEStGctsO1VhnbenJdFTb7Dieh4YcswaWRO8 ES8HVAq5e5UrT+Gz7y6JI9Hi0rtbA3/4cmDgil+JnmG5LyY1KJqt3Y6x6pd0G0U4UPaGl uXZ8w2qMw477ww805mzNDP+WFZqZrGiqV0jJiFCVYObdIVtHLGjRkCYjvUxLJ7XJ0V1Xg qyWIcBsqa66dfGizA6BGj2KdJuOPb/7RfsuRqezM0IePkqPhRO3sFHO/iw9w1mBJHqit1 aqVmGvmlHCQWsPW6xS4lZOzJVLGxo1/EekIZ+hg+F8ujs1AomGcm6FQDx0VBUk+VFKg== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794604190; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=Wtd2KDMKHjgdTYZDJFa9BcqH5dMoFb2/KH89jf9oLn0=; b=ioiErzlRunAJTdaCSK6FTdWrxdL9PPr2StxZZu1fu9qWreAv84vtQ/1KqDKEl3Xe7aRn Mwp7iYXlfDylIX/W4///igWAJisTKvhrDDi805HUKj2bbjlWizujScSDW5x5O7UJlBgks 2z4L3yXAvg76JAih7NPau9Mc2hjIno5dZC0/S29kHfL7tn+L3LUdW+HLAICRrcgKkiX9p YFvAx1tJ6nMvmYSLTbZWNEnb6zpYUQ9e4xJC7+KI16yKN/HiuahFQdl1rykt2NYsJyKfP g+bofQLVcpqXW6LFjb7KJnM0NViaVrvvrlKi4PG6wRAZUgJRyR+7wsWK0OvfAHDgnow== 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=bR7oYPNx; 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 DB66460A6F6E; Fri, 13 Nov 2026 21:09:50 +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=bR7oYPNx Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2591B1F000C4; Fri, 13 Nov 2026 21:09:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1794604190; bh=Wtd2KDMKHjgdTYZDJFa9BcqH5dMoFb2/KH89jf9oLn0=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=bR7oYPNxZlKgNdYEqiHrDBRrMaL2b5ONCmZ8lutbhmVhxtVH6WRk6YYJf42ZC2F7k WgZP5cHcDlKZ1Lfh6e3d946/KzOAXUgEUimc4GmgeO964LUTL48WkUwTYvTDilFhlZ InlQLb0ybNidY57Kk8+TjP/5L8I5ZSSb755JzsRL+9y0XhABWI5FsU2zQTE7bymW0x 5oy6xCBnqrV+m4hM7mpNS+2SAnZFo8sdVxbiiO/pI7f+6EXPGHG1zYhF9ckvZuZRtf 5q+WOwORKzCiqMk/+15c8Q0VXJ4SjcUCq1b7uGTybOYprX3I/upShUcBc0CiegBCq6 rywkHZwxP/kmA== Date: Fri, 13 Nov 2026 21:09:44 +0000 From: "Lorenzo Stoakes (ARM)" To: Tal Zussman Cc: "David Hildenbrand (Arm)" , 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, 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: linux-kernel@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: On Fri, Nov 13, 2026 at 03:55:41PM -0500, Tal Zussman wrote: > On 11/13/26 2:22 PM, Lorenzo Stoakes (ARM) wrote: > > 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 :) > > > This sounds better to me. My concern in [1] with completely disabling this > was that it could break a uffd manager that doesn't control the task it > manages (e.g. CRIU lazy restore registering the memory of a task whose libc > uses guard regions), but this should avoid that. But they were completely broken already? So we're 'breaking' an already broken user? :) It's moot, but still! > > This still leaves guard installation as the one zap path that doesn't send > a UFFD_EVENT_REMOVE notification, but for CRIU I think that's fine, since > guards would mostly go on memory that didn't have contents beforehand, so > there's no stale contents to restore. For one, CRIU is userland software that relies on kernel implementation details, so we are not obliged to keep _kernel_ implementation details stable for them :) For another, relying on uffd state remaining consistent across the install/remove of the literal 'put a hole in my address space' feature is deeply dubious. > > I could imagine this mattering for something like a userspace far-memory > library [2] that registers a heap with uffd MISSING mode. If the allocator > guards a page that the library has paged out, the library is never told > to drop its copy, and fills the page with the old contents once the guard > is removed. Previously: broken Now: Not permitted. I think that's fine ;) > > I don't think anything does this today, so it's not worth adding the event, > but I think this limitation should definitely be documented (i.e., that > MADV_GUARD_INSTALL discards the range and uffd state without notifying uffd) > for the sake of the poor souls that work with uffd and to prevent any > spurious LLM reports in the future... Documenting things doesn't prevent spurious LLM reports, unfortunately. Nothing stops the schloppers... ignoring is the only answer, long-term anyway. But sure, I'll try to update the manpage for the few humans still reading these things... > > [1] https://lore.kernel.org/linux-mm/f76cb2b0-0090-4554-9169-5cfff6a6ca53@columbia.edu/ > [2] https://www.usenix.org/system/files/nsdi25-yelam.pdf -- Cheers, Lorenzo