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Fri, 26 Jun 2020 11:26:04 +0000 (UTC) Date: Fri, 26 Jun 2020 12:26:02 +0100 From: "Dr. David Alan Gilbert" To: Vivek Goyal Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: drop Linux capabilities(7) Message-ID: <20200626112602.GG3087@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> <20200619113914.GI700896@redhat.com> <20200619191254.GH3154@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200619191254.GH3154@redhat.com> User-Agent: Mutt/1.14.3 (2020-06-14) 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=207.211.31.120; envelope-from=dgilbert@redhat.com; helo=us-smtp-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/26 01:49:42 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 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, Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , 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 12:39:14PM +0100, Daniel P. Berrangé wrote: > [..] > > The CAP_SYS_ADMIN requirement for 'trusted.' xattrs is simply a useful > > mechanism for applications to control access. The host kernel doesn' > > tuse this namespace itself. Linux has four namespaces for xattrs: > > > > - user - for userspace apps. accessible based on read/write permissions > > - trusted - for userspace apps. accessible by CAP_SYS_ADMIN processes only > > - system - for kernel only. used by ACLs > > - security - for kernel only. used by SELinux > > > > The use case for "trusted" xattrs is thus where a privileged management > > application or service wants to store metadata against the file, but > > also needs to grant an unprivileged process access to write to this file > > while not allowing that unprivileged process the ability to change the > > metadata. This is mentioned in the man page: > > > > [man xattr(7)] > > 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. > > > > User extended attributes > > User extended attributes may be assigned to files and directories > > for storing arbitrary additional information such as the mime type, > > character set or encoding of a file. The access permissions for > > user attributes are defined by the file permission bits: read per‐ > > mission is required to retrieve the attribute value, and writer per‐ > > mission is required to change it. > > [/man] > > > > Libvirtd uses the "trusted." xattr namespace to record information against > > disk images for QEMU, because we need to grant QEMU access to read/write > > the disk iamges, but don't want QEMU to be able to alter our xattrs. > > > > It is unfortunate that this namespace is tied to the CAP_SYS_ADMIN cap. > > It really ought to have had its own dedicated capability :-( Such is > > life with anything that uses CAP_SYS_ADMIN... > > > > With this in mind we really should have both trusted. & user. xattrs > > allowed to the guest by default. > > > > Conversely, we'll need to block usage of the security. and system. > > namespaces. > > I am wondering can we block usage of "system" and "security"? What > about guest setting acls over virtiofs files. These will have to > go through and that means we need to allow system xattrs. > > Similarly setting file capabilities inside should trigger > setxattr(security.capability) and that means we need to allow security > xattr as well. Yep, we see that when people install Fedora packages, when rpm unpackgs /usr/bin/newgidmap which has: $ getfattr -d '--match=.*' /usr/bin/newgidmap getfattr: Removing leading '/' from absolute path names # file: usr/bin/newgidmap security.capability=0sAQAAAkAAAAAAAAAAAAAAAAAAAAA= security.selinux="system_u:object_r:bin_t:s0" Dave > Thanks > Vivek -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK