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Fri, 19 Jun 2020 08:27:48 +0000 (UTC) Date: Fri, 19 Jun 2020 09:27:46 +0100 From: "Dr. David Alan Gilbert" To: Vivek Goyal Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: drop Linux capabilities(7) Message-ID: <20200619082746.GA2690@work-vm> References: <20200416164907.244868-1-stefanha@redhat.com> <20200618190816.GD3814@redhat.com> <20200618191655.GI2769@work-vm> <20200618192717.GE3814@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200618192717.GE3814@redhat.com> User-Agent: Mutt/1.14.0 (2020-05-02) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 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=205.139.110.61; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/19 02:45:39 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_H3=-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 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? Or what happens if I was to write random numbers into OVL_XATTR_NLINK? > > 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? > > 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. > 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. Dave > Thanks > Vivek -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK