From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E1C5E21ABBB; Fri, 23 Jan 2026 10:23:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769163835; cv=none; b=j0EtrwddiQlTQOqcxIgCCYxFUPrAK6B/EPoe346laqwm92k8nEmt00lm7V4Ubidl1yzu85LDpMdcZ/RC6gBTa/7gTKlWLa4SMHv8DEPrSOK+mR1uHu04V6HdEid4AV+scQYRsjwGb/vXUibz7ye4cygdMw3Ttuf3SOItYBvwlEA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769163835; c=relaxed/simple; bh=EHGi/yi88nJIt6UKAkgHIRx0Y1AOzHWxXOM0POlmDbs=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=stHDXiLBseEI3ehLZ++3jgAY2GVSSUQgIpTD4wIvsdXtuDVsuqW/KnelrmDY/C/VchNDs6MNYyUC4wgX36OIov5B1JVWvs+10j/e0pqj85dFngJ94L1Dx8SgQEKmgTy0csXmgxG5RdU6YrzALexqEMf56ZBrqIOc69MGEjo+50U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=owIHmuiN; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="owIHmuiN" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 34EB9C4CEF1; Fri, 23 Jan 2026 10:23:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1769163834; bh=EHGi/yi88nJIt6UKAkgHIRx0Y1AOzHWxXOM0POlmDbs=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=owIHmuiNnJn6LHl8oiUqHygfDSa6bV+gCvGGfscFbDPXXJnamr7kc99F49u7dqnTB mz2ieEOLcIEm+2lb+SWg5ZY9WmpZ2zvFsmpfwEj09P+14gdzhL7pSaB/XiSY2uhUHr zeAxux5Rm39+ruHliYMWB0Jb0qNo88k/0TtiD3kzIKdcYWKq/V9/KeDM4UszDznIdd w1Zcaq/AacA3FEpb+lEVD0Fj67fFb5BvGIqlLo9C4lWNHQT4NqaK3Qa2WEM3ObijYp WAxdIlthpVFMvzMYM02H+m0Y4+seYt3AfjKGFfo7Vakx/QhhAlUHvxArCBfABl1omB 9gbhSP4tNawFg== Date: Fri, 23 Jan 2026 11:23:46 +0100 From: Christian Brauner To: Andy Lutomirski Cc: Jeff Layton , Askar Safin , amir73il@gmail.com, cyphar@cyphar.com, jack@suse.cz, josef@toxicpanda.com, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, Lennart Poettering , David Howells , Yunkai Zhang , cgel.zte@gmail.com, Menglong Dong , linux-kernel@vger.kernel.org, initramfs@vger.kernel.org, containers@lists.linux.dev, linux-api@vger.kernel.org, news@phoronix.com, lwn@lwn.net, Jonathan Corbet , Rob Landley , emily@redcoat.dev, Christoph Hellwig Subject: Re: [PATCH 0/2] mount: add OPEN_TREE_NAMESPACE Message-ID: <20260123-autofrei-einspannen-7e65a6100e6e@brauner> References: <20251229-work-empty-namespace-v1-0-bfb24c7b061f@kernel.org> <20260119171101.3215697-1-safinaskar@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Jan 21, 2026 at 10:00:19AM -0800, Andy Lutomirski wrote: > > On Jan 19, 2026, at 2:21 PM, Jeff Layton wrote: > > > > On Mon, 2026-01-19 at 11:05 -0800, Andy Lutomirski wrote: > >>> On Mon, Jan 19, 2026 at 10:56 AM Askar Safin wrote: > >>> > >>> Christian Brauner : > >>>> Extend open_tree() with a new OPEN_TREE_NAMESPACE flag. Similar to > >>>> OPEN_TREE_CLONE only the indicated mount tree is copied. Instead of > >>>> returning a file descriptor referring to that mount tree > >>>> OPEN_TREE_NAMESPACE will cause open_tree() to return a file descriptor > >>>> to a new mount namespace. In that new mount namespace the copied mount > >>>> tree has been mounted on top of a copy of the real rootfs. > >>> > >>> I want to point at security benefits of this. > >>> > >>> [[ TL;DR: [1] and [2] are very big changes to how mount namespaces work. > >>> I like them, and I think they should get wider exposure. ]] > >>> > >>> If this patchset ([1]) and [2] both land (they are both in "next" now and > >>> likely will be submitted to mainline soon) and "nullfs_rootfs" is passed on > >>> command line, then mount namespace created by open_tree(OPEN_TREE_NAMESPACE) will > >>> usually contain exactly 2 mounts: nullfs and whatever was passed to > >>> open_tree(OPEN_TREE_NAMESPACE). > >>> > >>> This means that even if attacker somehow is able to unmount its root and > >>> get access to underlying mounts, then the only underlying thing they will > >>> get is nullfs. > >>> > >>> Also this means that other mounts are not only hidden in new namespace, they > >>> are fully absent. This prevents attacks discussed here: [3], [4]. > >>> > >>> Also this means that (assuming we have both [1] and [2] and "nullfs_rootfs" > >>> is passed), there is no anymore hidden writable mount shared by all containers, > >>> potentially available to attackers. This is concern raised in [5]: > >>> > >>>> You want rootfs to be a NULLFS instead of ramfs. You don't seem to want it to > >>>> actually _be_ a filesystem. Even with your "fix", containers could communicate > >>>> with each _other_ through it if it becomes accessible. If a container can get > >>>> access to an empty initramfs and write into it, it can ask/answer the question > >>>> "Are there any other containers on this machine running stux24" and then coordinate. > >> > >> I think this new OPEN_TREE_NAMESPACE is nifty, but I don't think the > >> path that gives it sensible behavior should be conditional like this. > >> Either make it *always* mount on top of nullfs (regardless of boot > >> options) or find some way to have it actually be the root. I assume > >> the latter is challenging for some reason. > >> > > > > I think that's the plan. I suggested the same to Christian last week, > > and he was amenable to removing the option and just always doing a > > nullfs_rootfs mount. > > > > We think that older runtimes should still "just work" with this scheme. > > Out of an abundance of caution, we _might_ want a command-line option > > to make it go back to old way, in case we find some userland stuff that > > doesn't like this for some reason, but hopefully we won't even need > > that. > > What I mean is: even if for some reason the kernel is running in a > mode where the *initial* rootfs is a real fs, I think it would be nice > for OPEN_TREE_NAMESPACE to use nullfs. The current patchset makes nullfs unconditional. As each mount namespaces creates a new copy of the namespace root of the namespace it was created from all mount namespace have nullfs as namespace root. So every OPEN_TREE_NAMESPACE/FSMOUNT_NAMESPACE will be mounted on top of nullfs as we always take the namespace root. If we have to make nullfs conditional then yes, we could still do that - althoug it would be ugly in various ways. I would love to keep nullfs unconditional because it means I can wipe a whole class of MNT_LOCKED nonsense from the face of the earth afterwards.