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 76C7E4409; Mon, 9 Dec 2024 17:15:41 +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=1733764541; cv=none; b=Ztu8DXcmsC/k7MpytBodug4RwXztqc8wrkPBvKDzlHeaImgUAeW5B7eExmNbhM93eFUdWRN6jb1WzFRpcjGEKUigQLsXg+/IlQbg8H/AXLAmczlxxS8TtvaCxo/YSMnUac68LiVRSUHgIFozleBbGtbmlJiHl89w5C6E4mkkGWQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1733764541; c=relaxed/simple; bh=3Cb+5lq/mU/71rJ8jtkSBxYaAnNkLJQcOfAJDDj4ngI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=nyBfwxMjRZTF9P72hOF9mvaZhIWzx2T/mNvye4OzaRmX6Nsb7MhXmwHwhk/WMr8MeIE0y8zuTIRa+pBpcYU6YwzJ11F1r8I2CZ8RfRUJ9VjgDDHOBk6V2/cNwFiCMAbXDvAK106fkDxIWmxSjEFL6iHriu3C30adfryQDw8/9eE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LdTeob6n; 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="LdTeob6n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA3C4C4CED1; Mon, 9 Dec 2024 17:15:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1733764540; bh=3Cb+5lq/mU/71rJ8jtkSBxYaAnNkLJQcOfAJDDj4ngI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=LdTeob6nWEuNpkAhvZ7ucYkCQSV3leJ7LRGglNHR6ZU2w/Lj9gvUC0Yr90iLnB+1G l7pIbqMDCunz50JzYizTmdJN9aI6boVhwPwejAqjoZTu+2pC2oZLFX34P3RplkYNLe 61VMerkooqSQ7ubkq4lvVGgi855gqx3zupRcv/nYJpNwuRzVRvwdTQCDuCeU5ok3n8 hUdwO67Cqo5DcEiP+UNoIhCWfjaGoZHKmS0PKN6YrwnYg9PdQ7JbAe+EIqalpX+KjE 859LcJ21KUF5zg8jpNWsFLbZqPxgIIbzMgD4e5Yhj62Imx8Ou77CAgkV6tjXOV2tLc x3wxpIerGpuKw== Message-ID: Subject: Re: [PATCH 0/4] exportfs: add flag to allow marking export operations as only supporting file handles From: Jeff Layton To: Chuck Lever , Amir Goldstein , Christoph Hellwig Cc: "Darrick J. Wong" , Christian Brauner , Erin Shepherd , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-nfs@vger.kernel.org, stable , Greg KH , Jens Axboe , Shaohua Li Date: Mon, 09 Dec 2024 12:15:38 -0500 In-Reply-To: <15628525-629f-49a4-a821-92092e2fa8cb@oracle.com> References: <20241201-work-exportfs-v1-0-b850dda4502a@kernel.org> <20241206160358.GC7820@frogsfrogsfrogs> <15628525-629f-49a4-a821-92092e2fa8cb@oracle.com> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 0OX6MYPtv76hka8lW4mnRmZqqx3UtfHX/hF/zH24Gj7A6sYKYLCU3YrI2Ogiu7/ksKcl7goQjpvtV YrOOI5VGLHge0awt7bhMCTM9KAfPc+xL/ZxAMVWd3NCk5SamL2cE99UWgtvNOIYU8m6EjTLhsj8sn 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wrote: > > > > To be clear, exporting pidfs or internal shmem via an anonymous fd = is > > > > probably not possible with existing userspace tools, but with all t= he new > > > > mount_fd and magic link apis, I can never be sure what can be made = possible > > > > to achieve when the user holds an anonymous fd. > > > >=20 > > > > The thinking behind adding the EXPORT_OP_LOCAL_FILE_HANDLE flag > > > > was that when kernfs/cgroups was added exportfs support with commit > > > > aa8188253474 ("kernfs: add exportfs operations"), there was no inte= ntion > > > > to export cgroupfs over nfs, only local to uses, but that was never= enforced, > > > > so we thought it would be good to add this restriction and backport= it to > > > > stable kernels. > > >=20 > > > Can you please explain what the problem with exporting these file > > > systems over NFS is? Yes, it's not going to be very useful. But wha= t > > > is actually problematic about it? Any why is it not problematic with > > > a userland nfs server? We really need to settle that argumet before > > > deciding a flag name or polarity. > > >=20 > >=20 > > I agree that it is not the end of the world and users do have to explic= itly > > use fsid=3D argument to be able to export cgroupfs via nfsd. > >=20 > > The idea for this patch started from the claim that Jeff wrote that cgr= oups > > is not allowed for nfsd export, but I couldn't find where it is not all= owed. > >=20 I think that must have been a wrong assumption on my part. I don't see anything that specifically prevents that either. If cgroupfs is mounted and you tell mountd to export it, I don't see what would prevent that. To be clear, I don't see how you would trick bog-standard mountd into exporting a filesystem that isn't mounted into its namespace, however. Writing a replacement for mountd is always a possibilty. > > I have no issue personally with leaving cgroupfs exportable via nfsd > > and changing restricting only SB_NOUSER and SB_KERNMOUNT fs. > >=20 > > Jeff, Chuck, what is your opinion w.r.t exportability of cgroupfs via n= fsd? >=20 > We all seem to be hard-pressed to find a usage scenario where exporting > pseudo-filesystems via NFS is valuable. But maybe someone has done it > and has a good reason for it. >=20 > The issue is whether such export should be consistently and actively > prevented. >=20 > I'm not aware of any specific security issues with it. >=20 >=20 I'm not either, but we are in new territory here. nfsd is a network service, so it does present more of an attack surface vs. local access. In general, you do have to take active steps to export a filesystem, but if someone exports / with "crossmnt", everything mounted is potentially accessible. That's obviously a dumb thing to do, but people make mistakes, and it's possible that doing this could be part of a wider exploit. I tend to think it safest to make exporting via nfsd an opt-in thing on a per-fs basis (along the lines of this patchset). If someone wants to allow access to more "exotic" filesystems, let them argue their use- case on the list first. --=20 Jeff Layton