Re: [PATCH 6/7] selinux: Ignore security labels on user namespace mounts

[Stephen Smalley <sds@tycho.nsa.gov>]

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 <seth.forshee@canonical.com>
>>>
>>> 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);