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

Stephen Smalley
2015-07-23T13:59:12Z

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 <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;
>
> 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.

>
>> + 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);
>>
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>
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