Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753907AbbGWPhu (ORCPT ); Thu, 23 Jul 2015 11:37:50 -0400 Received: from emvm-gh1-uea09.nsa.gov ([63.239.67.10]:62503 "EHLO emvm-gh1-uea09.nsa.gov" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752923AbbGWPhl (ORCPT ); Thu, 23 Jul 2015 11:37:41 -0400 X-TM-IMSS-Message-ID: Message-ID: <55B109E3.6030207@tycho.nsa.gov> Date: Thu, 23 Jul 2015 11:36:03 -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, Casey Schaufler 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> <55AFFFBD.8040907@tycho.nsa.gov> <55B0F2C0.8070709@tycho.nsa.gov> <20150723143920.GA25235@ubuntu-hedt> In-Reply-To: <20150723143920.GA25235@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: 4525 Lines: 90 On 07/23/2015 10:39 AM, Seth Forshee wrote: > On Thu, Jul 23, 2015 at 09:57:20AM -0400, Stephen Smalley wrote: >> On 07/22/2015 04:40 PM, Stephen Smalley wrote: >>> 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. >> >> Sorry, this won't work. bd_inode is not the inode of the block device >> file that was passed to mount, and it isn't labeled in any way. It will >> just be unlabeled. >> >> So I guess the only real option here as a fallback is >> sbsec->mntpoint_sid = current_sid(). Which isn't great either, as the >> only case where we currently assign task labels to files is for their >> /proc/pid inodes, and no current policy will therefore allow create >> permission to such files. > > Darn, you're right, that isn't the inode we want. There really doesn't > seem to be any way to get back to the one we want from the LSM, short of > adding a new hook. Maybe list_first_entry(&sb->s_bdev->bd_inodes, struct inode, i_devices)? Feels like a layering violation though... -- 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/