Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752912AbbGVU0q (ORCPT ); Wed, 22 Jul 2015 16:26:46 -0400 Received: from emvm-gh1-uea08.nsa.gov ([63.239.67.9]:64670 "EHLO emvm-gh1-uea08.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750779AbbGVU0o (ORCPT ); Wed, 22 Jul 2015 16:26:44 -0400 X-TM-IMSS-Message-ID: <386af3e700075995@nsa.gov> Message-ID: <55AFFC32.6070701@tycho.nsa.gov> Date: Wed, 22 Jul 2015 16:25:22 -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 , linux-kernel@vger.kernel.org, Andy Lutomirski , linux-security-module@vger.kernel.org, James Morris , linux-fsdevel@vger.kernel.org, selinux@tycho.nsa.gov, Alexander Viro 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> In-Reply-To: <20150722161422.GC124342@ubuntu-hedt> 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: 3891 Lines: 87 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; + 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); -- 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/