Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753462AbbGVUmE (ORCPT ); Wed, 22 Jul 2015 16:42:04 -0400 Received: from emvm-gh1-uea08.nsa.gov ([63.239.67.9]:50203 "EHLO emvm-gh1-uea08.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751990AbbGVUmA (ORCPT ); Wed, 22 Jul 2015 16:42:00 -0400 X-TM-IMSS-Message-ID: <3878ed5300075bee@nsa.gov> Message-ID: <55AFFFBD.8040907@tycho.nsa.gov> Date: Wed, 22 Jul 2015 16:40:29 -0400 From: Stephen Smalley Organization: National Security Agency User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Seth Forshee CC: Serge Hallyn , selinux@tycho.nsa.gov, linux-kernel@vger.kernel.org, Andy Lutomirski , linux-security-module@vger.kernel.org, Alexander Viro , James Morris , linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 6/7] selinux: Ignore security labels on user namespace mounts References: <1436989569-69582-1-git-send-email-seth.forshee@canonical.com> <1436989569-69582-7-git-send-email-seth.forshee@canonical.com> <55A7B055.4050809@tycho.nsa.gov> <55AFBE85.6010809@tycho.nsa.gov> <20150722161422.GC124342@ubuntu-hedt> <55AFFC32.6070701@tycho.nsa.gov> In-Reply-To: <55AFFC32.6070701@tycho.nsa.gov> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4359 Lines: 98 On 07/22/2015 04:25 PM, Stephen Smalley wrote: > On 07/22/2015 12:14 PM, Seth Forshee wrote: >> On Wed, Jul 22, 2015 at 12:02:13PM -0400, Stephen Smalley wrote: >>> On 07/16/2015 09:23 AM, Stephen Smalley wrote: >>>> On 07/15/2015 03:46 PM, Seth Forshee wrote: >>>>> Unprivileged users should not be able to supply security labels >>>>> in filesystems, nor should they be able to supply security >>>>> contexts in unprivileged mounts. For any mount where s_user_ns is >>>>> not init_user_ns, force the use of SECURITY_FS_USE_NONE behavior >>>>> and return EPERM if any contexts are supplied in the mount >>>>> options. >>>>> >>>>> Signed-off-by: Seth Forshee >>>> >>>> I think this is obsoleted by the subsequent discussion, but just for the >>>> record: this patch would cause the files in the userns mount to be left >>>> with the "unlabeled" label, and therefore under typical policies, >>>> completely inaccessible to any process in a confined domain. >>> >>> The right way to handle this for SELinux would be to automatically use >>> mountpoint labeling (SECURITY_FS_USE_MNTPOINT, normally set by >>> specifying a context= mount option), with the sbsec->mntpoint_sid set >>> from some related object (e.g. the block device file context, as in your >>> patches for Smack). That will cause SELinux to use that value instead >>> of any xattr value from the filesystem and will cause attempts by >>> userspace to set the security.selinux xattr to fail on that filesystem. >>> That is how SELinux normally deals with untrusted filesystems, except >>> that it is normally specified as a mount option by a trusted mounting >>> process, whereas in your case you need to automatically set it. >> >> Excellent, thank you for the advice. I'll start on this when I've >> finished with Smack. > > Not tested, but something like this should work. Note that it should > come after the call to security_fs_use() so we know whether SELinux > would even try to use xattrs supplied by the filesystem in the first place. > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index 564079c..84da3a2 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -745,6 +745,30 @@ static int selinux_set_mnt_opts(struct super_block *sb, > goto out; > } > } > + > + /* > + * If this is a user namespace mount, no contexts are allowed > + * on the command line and security labels must be ignored. > + */ > + if (sb->s_user_ns != &init_user_ns) { > + if (context_sid || fscontext_sid || rootcontext_sid || > + defcontext_sid) { > + rc = -EACCES; > + goto out; > + } > + if (sbsec->behavior == SECURITY_FS_USE_XATTR) { > + struct block_device *bdev = sb->s_bdev; > + sbsec->behavior = SECURITY_FS_USE_MNTPOINT; > + if (bdev) { > + struct inode_security_struct *isec = > bdev->bd_inode; That should be bdev->bd_inode->i_security. > + sbsec->mntpoint_sid = isec->sid; > + } else { > + sbsec->mntpoint_sid = current_sid(); > + } > + } > + goto out_set_opts; > + } > + > /* sets the context of the superblock for the fs being mounted. */ > if (fscontext_sid) { > rc = may_context_mount_sb_relabel(fscontext_sid, sbsec, > cred); > @@ -813,6 +837,7 @@ static int selinux_set_mnt_opts(struct super_block *sb, > sbsec->def_sid = defcontext_sid; > } > > +out_set_opts: > rc = sb_finish_set_opts(sb); > out: > mutex_unlock(&sbsec->lock); > > _______________________________________________ > Selinux mailing list > Selinux@tycho.nsa.gov > To unsubscribe, send email to Selinux-leave@tycho.nsa.gov. > To get help, send an email containing "help" to Selinux-request@tycho.nsa.gov. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/