From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 19 Jun 2020 17:16:48 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20200619161648.GJ2690@work-vm> References: <20200416164907.244868-1-stefanha@redhat.com> <20200618190816.GD3814@redhat.com> <20200618191655.GI2769@work-vm> <20200618192717.GE3814@redhat.com> <20200619082746.GA2690@work-vm> <20200619160923.GD3154@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20200619160923.GD3154@redhat.com> Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: drop Linux capabilities(7) List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Vivek Goyal Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org, Miklos Szeredi * Vivek Goyal (vgoyal@redhat.com) wrote: > On Fri, Jun 19, 2020 at 09:27:46AM +0100, Dr. David Alan Gilbert wrote: > > * Vivek Goyal (vgoyal@redhat.com) wrote: > > > On Thu, Jun 18, 2020 at 08:16:55PM +0100, Dr. David Alan Gilbert wrote: > > > > * Vivek Goyal (vgoyal@redhat.com) wrote: > > > > > On Thu, Apr 16, 2020 at 05:49:05PM +0100, Stefan Hajnoczi wrote: > > > > > > virtiofsd doesn't need of all Linux capabilities(7) available to root. Keep a > > > > > > whitelisted set of capabilities that we require. This improves security in > > > > > > case virtiofsd is compromised by making it hard for an attacker to gain further > > > > > > access to the system. > > > > > > > > > > Hi Stefan, > > > > > > > > > > I just noticed that this patch set breaks overlayfs on top of virtiofs. > > > > > > > > > > overlayfs sets "trusted.overlay.*" and xattrs in trusted domain > > > > > need CAP_SYS_ADMIN. > > > > > > > > > > man xattr says. > > > > > > > > > > Trusted extended attributes > > > > > Trusted extended attributes are visible and accessible only to pro‐ > > > > > cesses that have the CAP_SYS_ADMIN capability. Attributes in this > > > > > class are used to implement mechanisms in user space (i.e., outside the > > > > > kernel) which keep information in extended attributes to which ordinary > > > > > processes should not have access. > > > > > > > > > > There is a chance that overlay moves away from trusted xattr in future. > > > > > But for now we need to make it work. This is an important use case for > > > > > kata docker in docker build. > > > > > > > > > > May be we can add an option to virtiofsd say "--add-cap " and > > > > > ask user to pass in "--add-cap cap_sys_admin" if they need to run daemon > > > > > with this capaibility. > > > > > > > > I'll admit I don't like the idea of giving it cap_sys_admin. > > > > Can you explain: > > > > a) What overlayfs uses trusted for? > > > > > > overlayfs stores bunch of metadata and uses "trusted" xattrs for it. > > > > Tell me more about this metadata. > > Taking a juicy looking one, what does OVL_XATTR_REDIRECT do? > > It contains path information which is used for lookup into lower layer. > > > Or what happens if I was to write random numbers into OVL_XATTR_NLINK? > > Overlay is storing its metadata in trusted.* xattrs. If a user modifies > metadata, then various kind of bad things can happen. I think one can > do some kind of checks on metadata to make sure it does not crash > atleast. > > And that's true for any filesystem. Isn't. If user manages to modify > metadata outside of filesystem, then lot of bad things can happen. I > thought that's the reason that people are not comfortable with the > idea of allowing mounting filesystem from inside user namespace because > it makes it easy to mount a hand crafted filesystem. > > Anyway, I think overlayfs is just one use case of trusted xattr. Even > if overlayfs moves away from trusted xattr, what about other users. > We need to have a story around how will we support trusted xattrs > safely. > > > > > > > > b) If something nasty was to write junk into the trusted attributes, > > > > what would happen? > > > > > > This directory is owned by guest. So it should be able to write > > > anything it wants, as long as process in guest has CAP_SYS_ADMIN, right? > > > > Well, we shouldn't be able to break/crash/escape into the host; how > > much does overlayfs validate trusted.* it uses? > > I thought qemu and kvm are the one who should ensure we should not be > able to break out of sandbox. Kernel implementation could be as > buggy as it wanted to be. We are working with this security model > that kernel is completely untrusted. But with virtiofs we allow the guest to do a lot of filesystem operations on the host. It's the virtiofsd that has to ensure that these are safe and contained within the fs it's exposed; the qemu/kvm can't protect us from that. That's why we sandbox the virtiofsd like we do - if we allow a priviliged guest to perform calls to an unconstrained virtiofsd it would be able to escape. That's what I want to check. Dave > > > > > > c) I see overlayfs has a fallback check if xattr isn't supported at > > > > all - what is the consequence? > > > > > > It falls back to I think read only mode. > > > > It looks like the fallback is more subtle to me: > > /* > > * Check if upper/work fs supports trusted.overlay.* xattr > > */ > > err = ovl_do_setxattr(ofs->workdir, OVL_XATTR_OPAQUE, "0", 1, 0); > > if (err) { > > ofs->noxattr = true; > > ofs->config.index = false; > > ofs->config.metacopy = false; > > pr_warn("upper fs does not support xattr, falling back to index=off and metacopy=off.\n"); > > > > but I don't know what index and metacopy are. > > They enable certain features in overlayfs. In fact, we fall back to > lesser capability on if we are running on ext4/xfs. For virtiofs, > we deny the mount completely. > > /* > * We allowed sub-optimal upper fs configuration and don't want to break > * users over kernel upgrade, but we never allowed remote upper fs, so > * we can enforce strict requirements for remote upper fs. > */ > if (ovl_dentry_remote(ofs->workdir) && > (!d_type || !rename_whiteout || ofs->noxattr)) { > pr_err("upper fs missing required features.\n"); > err = -EINVAL; > goto out; > } > > > > > > For a moment forget about overlayfs. Say a user process in guest with > > > CAP_SYS_ADMIN is writing trusted.foo. Should that succeed? Is a > > > passthrough filesystem, so it should go through. But currently it > > > wont. > > > > As long as any effects of what it writes are contained to the area of > > the filesystem exposed to the guest, yes - however it worries me what > > the consequences of broken trusted metadata is. If it's delicate enough > > that it's guarded by CAP_SYS_ADMIN someone must have worried about it. > > Agreed that we need to look into whether having CAP_SYS_ADMIN allow > virtiofsd to break out of jail. > > May be we need to provide that remapping trusted xattr feature so > that we don't have to have CAP_SYS_ADMIN in init_user_ns and can > provide this emulation even when running in user namespace. > > Vivek -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9339FC433E1 for ; Fri, 19 Jun 2020 16:17:59 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 630242168B for ; Fri, 19 Jun 2020 16:17:59 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ZQbnlHjg" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 630242168B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:56128 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1jmJiI-0001VR-Mc for qemu-devel@archiver.kernel.org; Fri, 19 Jun 2020 12:17:58 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:43680) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1jmJhO-00013U-Rt for qemu-devel@nongnu.org; Fri, 19 Jun 2020 12:17:02 -0400 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:31568 helo=us-smtp-1.mimecast.com) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1jmJhM-0003mF-Ta for qemu-devel@nongnu.org; Fri, 19 Jun 2020 12:17:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592583420; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=L1EmuAtqu5wC9RUoXSdxE0F5jSeteEB0wjH+slg6H/8=; b=ZQbnlHjgQBFfqNq0v4+TDS+Fh160KO5X/rnohLKdYSeWnUB/CXqjyu68aTMrgPKrW0amSF aHHrIRItdgbl/YPP+pZkXIgPMUB+hhjmzanqCaTRcYHfEC5s3ExW4MiYUTgZmS0a8gP+xk AczSNLF97ugwP5EvwkY5+IP+AM32OtA= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-202-iQU8gNMAOXaAq-WFuF5tdw-1; Fri, 19 Jun 2020 12:16:58 -0400 X-MC-Unique: iQU8gNMAOXaAq-WFuF5tdw-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7E7AA81A3FF; Fri, 19 Jun 2020 16:16:57 +0000 (UTC) Received: from work-vm (ovpn-114-215.ams2.redhat.com [10.36.114.215]) by smtp.corp.redhat.com (Postfix) with ESMTPS id E37FF71660; Fri, 19 Jun 2020 16:16:50 +0000 (UTC) Date: Fri, 19 Jun 2020 17:16:48 +0100 From: "Dr. David Alan Gilbert" To: Vivek Goyal Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: drop Linux capabilities(7) Message-ID: <20200619161648.GJ2690@work-vm> References: <20200416164907.244868-1-stefanha@redhat.com> <20200618190816.GD3814@redhat.com> <20200618191655.GI2769@work-vm> <20200618192717.GE3814@redhat.com> <20200619082746.GA2690@work-vm> <20200619160923.GD3154@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200619160923.GD3154@redhat.com> User-Agent: Mutt/1.14.0 (2020-05-02) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/19 05:19:18 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=-0.01, RCVD_IN_MSPIKE_WL=-0.01, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: virtio-fs@redhat.com, qemu-devel@nongnu.org, Stefan Hajnoczi , Miklos Szeredi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Vivek Goyal (vgoyal@redhat.com) wrote: > On Fri, Jun 19, 2020 at 09:27:46AM +0100, Dr. David Alan Gilbert wrote: > > * Vivek Goyal (vgoyal@redhat.com) wrote: > > > On Thu, Jun 18, 2020 at 08:16:55PM +0100, Dr. David Alan Gilbert wrote: > > > > * Vivek Goyal (vgoyal@redhat.com) wrote: > > > > > On Thu, Apr 16, 2020 at 05:49:05PM +0100, Stefan Hajnoczi wrote: > > > > > > virtiofsd doesn't need of all Linux capabilities(7) available to root. Keep a > > > > > > whitelisted set of capabilities that we require. This improves security in > > > > > > case virtiofsd is compromised by making it hard for an attacker to gain further > > > > > > access to the system. > > > > > > > > > > Hi Stefan, > > > > > > > > > > I just noticed that this patch set breaks overlayfs on top of virtiofs. > > > > > > > > > > overlayfs sets "trusted.overlay.*" and xattrs in trusted domain > > > > > need CAP_SYS_ADMIN. > > > > > > > > > > man xattr says. > > > > > > > > > > Trusted extended attributes > > > > > Trusted extended attributes are visible and accessible only to pro‐ > > > > > cesses that have the CAP_SYS_ADMIN capability. Attributes in this > > > > > class are used to implement mechanisms in user space (i.e., outside the > > > > > kernel) which keep information in extended attributes to which ordinary > > > > > processes should not have access. > > > > > > > > > > There is a chance that overlay moves away from trusted xattr in future. > > > > > But for now we need to make it work. This is an important use case for > > > > > kata docker in docker build. > > > > > > > > > > May be we can add an option to virtiofsd say "--add-cap " and > > > > > ask user to pass in "--add-cap cap_sys_admin" if they need to run daemon > > > > > with this capaibility. > > > > > > > > I'll admit I don't like the idea of giving it cap_sys_admin. > > > > Can you explain: > > > > a) What overlayfs uses trusted for? > > > > > > overlayfs stores bunch of metadata and uses "trusted" xattrs for it. > > > > Tell me more about this metadata. > > Taking a juicy looking one, what does OVL_XATTR_REDIRECT do? > > It contains path information which is used for lookup into lower layer. > > > Or what happens if I was to write random numbers into OVL_XATTR_NLINK? > > Overlay is storing its metadata in trusted.* xattrs. If a user modifies > metadata, then various kind of bad things can happen. I think one can > do some kind of checks on metadata to make sure it does not crash > atleast. > > And that's true for any filesystem. Isn't. If user manages to modify > metadata outside of filesystem, then lot of bad things can happen. I > thought that's the reason that people are not comfortable with the > idea of allowing mounting filesystem from inside user namespace because > it makes it easy to mount a hand crafted filesystem. > > Anyway, I think overlayfs is just one use case of trusted xattr. Even > if overlayfs moves away from trusted xattr, what about other users. > We need to have a story around how will we support trusted xattrs > safely. > > > > > > > > b) If something nasty was to write junk into the trusted attributes, > > > > what would happen? > > > > > > This directory is owned by guest. So it should be able to write > > > anything it wants, as long as process in guest has CAP_SYS_ADMIN, right? > > > > Well, we shouldn't be able to break/crash/escape into the host; how > > much does overlayfs validate trusted.* it uses? > > I thought qemu and kvm are the one who should ensure we should not be > able to break out of sandbox. Kernel implementation could be as > buggy as it wanted to be. We are working with this security model > that kernel is completely untrusted. But with virtiofs we allow the guest to do a lot of filesystem operations on the host. It's the virtiofsd that has to ensure that these are safe and contained within the fs it's exposed; the qemu/kvm can't protect us from that. That's why we sandbox the virtiofsd like we do - if we allow a priviliged guest to perform calls to an unconstrained virtiofsd it would be able to escape. That's what I want to check. Dave > > > > > > c) I see overlayfs has a fallback check if xattr isn't supported at > > > > all - what is the consequence? > > > > > > It falls back to I think read only mode. > > > > It looks like the fallback is more subtle to me: > > /* > > * Check if upper/work fs supports trusted.overlay.* xattr > > */ > > err = ovl_do_setxattr(ofs->workdir, OVL_XATTR_OPAQUE, "0", 1, 0); > > if (err) { > > ofs->noxattr = true; > > ofs->config.index = false; > > ofs->config.metacopy = false; > > pr_warn("upper fs does not support xattr, falling back to index=off and metacopy=off.\n"); > > > > but I don't know what index and metacopy are. > > They enable certain features in overlayfs. In fact, we fall back to > lesser capability on if we are running on ext4/xfs. For virtiofs, > we deny the mount completely. > > /* > * We allowed sub-optimal upper fs configuration and don't want to break > * users over kernel upgrade, but we never allowed remote upper fs, so > * we can enforce strict requirements for remote upper fs. > */ > if (ovl_dentry_remote(ofs->workdir) && > (!d_type || !rename_whiteout || ofs->noxattr)) { > pr_err("upper fs missing required features.\n"); > err = -EINVAL; > goto out; > } > > > > > > For a moment forget about overlayfs. Say a user process in guest with > > > CAP_SYS_ADMIN is writing trusted.foo. Should that succeed? Is a > > > passthrough filesystem, so it should go through. But currently it > > > wont. > > > > As long as any effects of what it writes are contained to the area of > > the filesystem exposed to the guest, yes - however it worries me what > > the consequences of broken trusted metadata is. If it's delicate enough > > that it's guarded by CAP_SYS_ADMIN someone must have worried about it. > > Agreed that we need to look into whether having CAP_SYS_ADMIN allow > virtiofsd to break out of jail. > > May be we need to provide that remapping trusted xattr feature so > that we don't have to have CAP_SYS_ADMIN in init_user_ns and can > provide this emulation even when running in user namespace. > > Vivek -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK