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From: Stephen Smalley <sds@tycho.nsa.gov>
To: Daniel Colascione <dancol@google.com>
Cc: Tim Murray <timmurray@google.com>,
	Nosh Minwalla <nosh@google.com>, Nick Kralevich <nnk@google.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	selinux@vger.kernel.org
Subject: Re: [PATCH v2 3/6] Teach SELinux about a new userfaultfd class
Date: Wed, 12 Feb 2020 14:17:23 -0500
Message-ID: <94b0d8bf-9990-6d3d-e9a6-aa761f191e55@tycho.nsa.gov> (raw)
In-Reply-To: <626a2302-5b5f-d7c1-fdef-51094bb1fe0d@tycho.nsa.gov>

On 2/12/20 2:11 PM, Stephen Smalley wrote:
> On 2/12/20 2:04 PM, Daniel Colascione wrote:
>> On Wed, Feb 12, 2020 at 10:59 AM Stephen Smalley <sds@tycho.nsa.gov> 
>> wrote:
>>>
>>> On 2/12/20 1:04 PM, Stephen Smalley wrote:
>>>> On 2/12/20 12:19 PM, Daniel Colascione wrote:
>>>>> Thanks for taking a look.
>>>>>
>>>>> On Wed, Feb 12, 2020 at 9:04 AM Stephen Smalley <sds@tycho.nsa.gov>
>>>>> wrote:
>>>>>>
>>>>>> On 2/11/20 5:55 PM, Daniel Colascione wrote:
>>>>>>> Use the secure anonymous inode LSM hook we just added to let SELinux
>>>>>>> policy place restrictions on userfaultfd use. The create operation
>>>>>>> applies to processes creating new instances of these file objects;
>>>>>>> transfer between processes is covered by restrictions on read, 
>>>>>>> write,
>>>>>>> and ioctl access already checked inside selinux_file_receive.
>>>>>>>
>>>>>>> Signed-off-by: Daniel Colascione <dancol@google.com>
>>>>>>
>>>>>> (please add linux-fsdevel and viro to the cc for future versions 
>>>>>> of this
>>>>>> patch since it changes the VFS)
>>>>>>
>>>>>>> ---
>>>>>>> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
>>>>>>> index 1659b59fb5d7..e178f6f40e93 100644
>>>>>>> --- a/security/selinux/hooks.c
>>>>>>> +++ b/security/selinux/hooks.c
>>>>>>> @@ -2915,6 +2919,69 @@ static int selinux_inode_init_security(struct
>>>>>>> inode *inode, struct inode *dir,
>>>>>>> +
>>>>>>> +     /*
>>>>>>> +      * We shouldn't be creating secure anonymous inodes before LSM
>>>>>>> +      * initialization completes.
>>>>>>> +      */
>>>>>>> +     if (unlikely(!selinux_state.initialized))
>>>>>>> +             return -EBUSY;
>>>>>>
>>>>>> I don't think this is viable; any arbitrary actions are possible 
>>>>>> before
>>>>>> policy is loaded, and a Linux distro can be brought up fully with
>>>>>> SELinux enabled and no policy loaded.  You'll just need to have a
>>>>>> default behavior prior to initialization.
>>>>>
>>>>> We'd have to fail open then, I think, and return an S_PRIVATE inode
>>>>> (the regular anon inode).
>>>>
>>>> Not sure why.  You aren't doing anything in the hook that actually
>>>> relies on selinux_state.initialized being set (i.e. nothing requires a
>>>> policy).  The avc_has_perm() call will just succeed until a policy is
>>>> loaded.  So if these inodes are created prior to policy load, they will
>>>> get assigned the task SID (which would be the kernel SID prior to 
>>>> policy
>>>> load or first exec or write to /proc/self/attr/current afterward) and
>>>> UFFD class (in your current code), be permitted, and then once 
>>>> policy is
>>>> loaded any further access will get checked against the kernel SID.
>>>>
>>>>>>> +     /*
>>>>>>> +      * We only get here once per ephemeral inode.  The inode has
>>>>>>> +      * been initialized via inode_alloc_security but is otherwise
>>>>>>> +      * untouched, so check that the state is as
>>>>>>> +      * inode_alloc_security left it.
>>>>>>> +      */
>>>>>>> +     BUG_ON(isec->initialized != LABEL_INVALID);
>>>>>>> +     BUG_ON(isec->sclass != SECCLASS_FILE);
>>>>>>
>>>>>> I think the kernel discourages overuse of BUG_ON/BUG/...
>>>>>
>>>>> I'm not sure what counts as overuse.
>>>>
>>>> Me either (not my rule) but I'm pretty sure this counts or you'd see a
>>>> lot more of these kinds of BUG_ON() checks throughout.  Try to reserve
>>>> them for really critical cases.
>>>>
>>>>>>> +
>>>>>>> +#ifdef CONFIG_USERFAULTFD
>>>>>>> +     if (fops == &userfaultfd_fops)
>>>>>>> +             isec->sclass = SECCLASS_UFFD;
>>>>>>> +#endif
>>>>>>
>>>>>> Not sure we want or need to introduce a new security class for 
>>>>>> each user
>>>>>> of anonymous inodes since the permissions should be the same as for
>>>>>> file.
>>>>>
>>>>> The purpose of this change is to apply special policy to userfaultfd
>>>>> FDs in particular. Isn't having a UFFD security class the best way to
>>>>> go about that? (There's no path.) Am I missing something?
>>>>
>>>> It is probably the simplest approach; it just doesn't generalize to all
>>>> users of anonymous inodes. We can distinguish them in one of two ways:
>>>> use a different class like you did (requires a code change every 
>>>> time we
>>>> add a new one and yet another duplicate of the file class) or use a
>>>> different SID/context/type. The latter could be achieved by calling
>>>> security_transition_sid() with the provided name wrapped in a qstr and
>>>> specifying type_transition rules on the name.  Then policy could define
>>>> derived types for each domain, ala
>>>> type_transition init self:file "[userfaultfd]" init_userfaultfd;
>>>> type_transition untrusted_app self:file "[userfaultfd]"
>>>> untrusted_app_userfaultfd;
>>>> ...
>>>>
>>>>>> Also not sure we want to be testing fops for each such case.
>>>>>
>>>>> I was also thinking of just providing some kind of context string
>>>>> (maybe the name), which might be friendlier to modules, but the loose
>>>>> coupling kind of scares me, and for this particular application, since
>>>>> UFFD is always in the core and never in a module, checking the fops
>>>>> seems a bit more robust and doesn't hurt anything.
>>>>
>>>> Yes, not sure how the vfs folks feel about either coupling (the
>>>> name-based one or the fops-based one).  Neither seems great.
>>>>
>>>>>> We
>>>>>> were looking at possibly leveraging the name as a key and using
>>>>>> security_transition_sid() to generate a distinct SID/context/type for
>>>>>> the inode via type_transition rules in policy.  We have some WIP 
>>>>>> along
>>>>>> those lines.
>>>>>
>>>>> Where? Any chance it would be ready soon? I'd rather not hold up this
>>>>> work for a more general mechanism.
>>>>
>>>> Hopefully will have a patch available soon.  But not saying this
>>>> necessarily has to wait either.
>>>>
>>>>>>> +     /*
>>>>>>> +      * Always give secure anonymous inodes the sid of the
>>>>>>> +      * creating task.
>>>>>>> +      */
>>>>>>> +
>>>>>>> +     isec->sid = tsec->sid;
>>>>>>
>>>>>> This doesn't generalize for other users of anonymous inodes, e.g. the
>>>>>> /dev/kvm case where we'd rather inherit the SID and class from the
>>>>>> original /dev/kvm inode itself.
>>>>>
>>>>> I think someone mentioned on the first version of this patch that we
>>>>> could make it more flexible if the need arose. If we do want to do it
>>>>> now, we could have the anon_inode security hook accept a "parent" or
>>>>> "context" inode that modules could inspect for the purposes of forming
>>>>> the new inode's SID. Does that make sense to you?
>>>>
>>>> Yes, that's the approach in our current WIP, except we call it a
>>>> "related" inode since it isn't necessarily connected to the anon inode
>>>> in any vfs sense.
>>>
>>> The other key difference in our WIP approach is that we assumed that we
>>> couldn't mandate allocating a separate anon inode for each of these fds
>>> and we wanted to cover all anonymous inodes (not opt-in), so we are
>>> storing the SID/class pair as additional fields in the
>>> file_security_struct and have modified file_has_perm() and others to
>>> look there for anonymous inodes.
>>
>> A separate inode seems like the simpler approach for now, because it
>> means that we have fewer places to check for security information ---
>> and it's not as if an inode is particularly expensive. We can always
>> switch later.
> 
> We'd prefer having a separate inode if possible but didn't think that 
> would fly with the vfs folks, especially if we try to apply this to all 
> anonymous inodes. It might be ok for userfaultfd usage as a specific 
> case but there is a reason why anonymous inodes were introduced and 
> creating a separate inode each time defeats that purpose IIUC.  It will 
> be interesting to see how they respond.

I suppose an optimization of your approach could be to only allocate a 
new anon inode if there isn't already one that has the same security 
info (SID/class pair in the SELinux case).

  parent reply index

Thread overview: 75+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-11 22:55 [PATCH v2 0/6] Harden userfaultfd Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 1/6] Add a new flags-accepting interface for anonymous inodes Daniel Colascione
2020-02-12 16:37   ` Stephen Smalley
2020-02-12 17:23     ` Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 2/6] Add a concept of a "secure" anonymous file Daniel Colascione
2020-02-12 16:49   ` Stephen Smalley
2020-02-14 22:13   ` kbuild test robot
2020-02-11 22:55 ` [PATCH v2 3/6] Teach SELinux about a new userfaultfd class Daniel Colascione
2020-02-12 17:05   ` Stephen Smalley
2020-02-12 17:19     ` Daniel Colascione
2020-02-12 18:04       ` Stephen Smalley
2020-02-12 18:59         ` Stephen Smalley
2020-02-12 19:04           ` Daniel Colascione
2020-02-12 19:11             ` Stephen Smalley
2020-02-12 19:13               ` Daniel Colascione
2020-02-12 19:17               ` Stephen Smalley [this message]
2020-02-11 22:55 ` [PATCH v2 4/6] Wire UFFD up to SELinux Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 5/6] Let userfaultfd opt out of handling kernel-mode faults Daniel Colascione
2020-02-11 22:55 ` [PATCH v2 6/6] Add a new sysctl for limiting userfaultfd to user mode faults Daniel Colascione
2020-02-11 23:13 ` [PATCH v2 0/6] Harden userfaultfd Casey Schaufler
2020-02-11 23:27   ` Daniel Colascione
2020-02-12 16:09     ` Stephen Smalley
2020-02-21 17:56     ` James Morris
2020-02-12  7:50 ` Kees Cook
2020-02-12 16:54   ` Jann Horn
2020-02-12 17:14     ` Peter Xu
2020-02-12 19:41       ` Andrea Arcangeli
2020-02-12 20:04         ` Daniel Colascione
2020-02-12 23:41           ` Andrea Arcangeli
2020-02-12 17:12   ` Daniel Colascione
2020-02-14  3:26 ` [PATCH 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-02-14  3:26   ` [PATCH 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-02-14  3:26   ` [PATCH 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-02-14 16:39     ` Stephen Smalley
2020-02-14 17:21       ` Daniel Colascione
2020-02-14 18:02         ` Stephen Smalley
2020-02-14 18:08           ` Stephen Smalley
2020-02-14 20:24             ` Stephen Smalley
2020-02-14  3:26   ` [PATCH 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:02   ` [PATCH v2 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-25 23:02   ` [PATCH v2 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 13:53     ` Stephen Smalley
2020-03-25 23:02   ` [PATCH v2 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 13:58     ` Stephen Smalley
2020-03-26 17:59       ` Daniel Colascione
2020-03-26 17:37     ` Stephen Smalley
2020-03-25 23:02   ` [PATCH v2 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-25 23:49     ` Casey Schaufler
2020-03-26 18:14   ` [PATCH v3 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 18:14     ` [PATCH v3 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-26 19:00       ` Stephen Smalley
2020-03-26 18:14     ` [PATCH v3 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-26 19:02       ` Stephen Smalley
2020-03-26 18:14     ` [PATCH v3 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-03-26 20:06     ` [PATCH v4 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-03-26 20:06       ` [PATCH v4 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-03-27 13:40         ` Stephen Smalley
2020-03-26 20:06       ` [PATCH v4 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-03-27 13:41         ` Stephen Smalley
2020-03-26 20:06       ` [PATCH v4 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-04-01 21:39       ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-01 21:39         ` [PATCH v5 1/3] Add a new LSM-supporting anonymous inode interface Daniel Colascione
2020-05-07 16:02           ` James Morris
2020-04-01 21:39         ` [PATCH v5 2/3] Teach SELinux about anonymous inodes Daniel Colascione
2020-04-01 21:39         ` [PATCH v5 3/3] Wire UFFD up to SELinux Daniel Colascione
2020-04-13 13:29         ` [PATCH v5 0/3] SELinux support for anonymous inodes and UFFD Daniel Colascione
2020-04-22 16:55           ` James Morris
2020-04-22 17:12             ` Casey Schaufler
2020-04-23 22:24               ` Casey Schaufler
2020-04-27 16:18                 ` Casey Schaufler
2020-04-27 16:48                   ` Stephen Smalley
2020-04-27 17:12                     ` Casey Schaufler
2020-04-29 17:02                     ` Stephen Smalley
2020-04-27 17:15             ` Casey Schaufler
2020-04-27 19:40               ` Stephen Smalley

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