Re: [kernel-hardening] Re: [PATCH v7 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

[Matt Brown <matt@nmatt.com>]

On 6/2/17 12:57 PM, Serge E. Hallyn wrote:
> Quoting Matt Brown (matt@nmatt.com):
>> On 6/2/17 11:36 AM, Serge E. Hallyn wrote:
>>> Quoting Matt Brown (matt@nmatt.com):
>>>> On 6/1/17 5:24 PM, Alan Cox wrote:
>>>>>> There's a difference between "bugs" and "security bugs". Letting
>>>>>
>>>>> Not really, it's merely a matter of severity of result. A non security
>>>>> bug that hoses your hard disk is to anyone but security nutcases at
>>>>> least as bad as a security hole.
>>>>>
>>>>>> security bugs continue to get exploited because we want to flush out
>>>>>> bugs seems insensitive to the people getting attacked. I'd rather
>>>>>> protect against a class of bug than have to endless fix each bug.
>>>>>
>>>>> The others are security bugs too to varying degree
>>>>>
>>>>>>> I'm not against doing something to protect the container folks, but that
>>>>>>> something as with Android is a whitelist of ioctls. And if we need to do
>>>>>>> this with a kernel hook lets do it properly.
>>>>>>>
>>>>>>> Remember the namespace of the tty on creation
>>>>>>> If the magic security flag is set then
>>>>>>>         Apply a whitelist to *any* tty ioctl call where the ns doesn't
>>>>>>>                 match
>>>>>>>
>>>>>>> and we might as well just take the Android whitelist since they've kindly
>>>>>>> built it for us all!
>>>>>>>
>>>>>>> In the tty layer it ends up being something around 10 lines of code and
>>>>>>> some other file somewhere in security/ that's just a switch or similar
>>>>>>> with the whitelisted ioctl codes in it.
>>>>>>>
>>>>>>> That (or a similar SELinux ruleset) would actually fix the problem.
>>>>>>> SELinux would be better because it can also apply the rules when doing
>>>>>>> things like su/sudo/...  
>>>>>>
>>>>>> Just to play devil's advocate, wouldn't such a system continue to not
>>>>>> address your physical-console concerns? I wouldn't want to limit the
>>>>>
>>>>> It would for the cases that a whitelist and container check covers -
>>>>> because the whitelist wouldn't allow you to do anything but boring stuff
>>>>> on the tty. TIOCSTI is just one of a whole range of differently stupid
>>>>> and annoying opportunities. Containers do not and should not be able to
>>>>> set the keymap, change the video mode, use console selection, make funny
>>>>> beepy noises, access video I/O registers and all the other stuff like
>>>>> that. Nothing is going to break if we have a fairly conservative
>>>>> whitelist.
>>>>>
>>>>>> protection to only containers (but it's a good start), since it
>>>>>> wouldn't protect people not using containers that still have a
>>>>>> privileged TTY attached badly somewhere.
>>>>>
>>>>> How are you going to magically fix the problem. I'm not opposed to fixing
>>>>> the real problem but right now it appears to be a product of wishful
>>>>> thinking not programming. What's the piece of security code that
>>>>> magically discerns the fact you are running something untrusted at the
>>>>> other end of your tty. SELinux can do it via labelling but I don't see
>>>>> any generic automatic way for the kernel to magically work out when to
>>>>> whitelist and when not to. If there is a better magic rule than
>>>>> differing-namespace then provide the code.
>>>>>
>>>>> You can't just disable TIOCSTI, it has users deal with it. You can
>>>>> get away with disabling it for namespace crossing I think but if you do
>>>>> that you need to disable a pile of others.
>>>>>
>>>>> (If it breaks containers blocking TIOCSTI then we need to have a good
>>>>> look at algorithms for deciding when to flush the input queue on exiting
>>>>> a container or somesuch)
>>>>>
>>>>>> If you're talking about wholistic SELinux policy, sure, I could
>>>>>> imagine a wholistic fix. But for the tons of people without a
>>>>>> comprehensive SELinux policy, the proposed protection continues to
>>>>>> make sense.
>>>>>
>>>>> No it doesn't. It's completely useless unless you actually bother to
>>>>> address the other exploit opportunities.
>>>>>
>>>>> Right now the proposal is a hack to do 
>>>>>
>>>>> 	if (TIOCSTI && different_namespace && magic_flag)
>>>>>
>>>>
>>>>
>>>> This is not what my patch does. Mine is like:
>>>>
>>>> 	if (TIOCSTI && !ns_capable(tty->owner_user_ns, CAP_SYS_ADMIN) &&
>>>> 		magic_flag)
>>>>
>>>> in other words:
>>>> 	if (TIOCSTI && (different_owner_user_ns || !CAP_SYS_ADMIN) &&
>>>> 		magic_flag)
>>>>
>>>> can you specify what you mean by different_namespace? which namespace?
>>>
>>> I think you're focusing on the wrong thing.  Your capable check (apart
>>> from the fact that I think I've been convinced CAP_SYS_ADMIN is wrong)
>>> is fine.
>>
> 
> I'm cc:ing linux-api here because really we're designing an interesting
> API.
> 
>> Can't we also have a sysctl that toggles if CAP_SYS_ADMIN is involved in
>> this whitelist check?  Otherwise someone might leave things out of the
>> whitelist just because they want to use those ioctls as a privileged
>> process.
> 
> I'm not quite sure what you're asking for here.  Let me offer a precise
> strawman design.  I'm sure there are problems with it, it's just a starting
> point.
> 
> system-wide whitelist (for now 'may_push_chars') is full by default.
> 

So is may_push_chars just an alias for TIOCSTI? Or are there some
potential whitelist members that would map to multiple ioctls?

> By default, nothing changes - you can use those on your own tty, need
> CAP_SYS_ADMIN against init_user_ns otherwise.
> 
> Introduce a new CAP_TTY_PRIVILEGED.
> 

I'm fine with this.

> When may_push_chars is removed from the whitelist, you lose the ability
> to use TIOCSTI on a tty - even your own - if you do not have CAP_TTY_PRIVILEGED
> against the tty's user_ns.
> 

How do you propose storing/updating the whitelist? sysctl?

If it is a sysctl, would each whitelist member have a sysctl?
e.g.: kernel.ioctlwhitelist.may_push_chars = 1

Overall, I'm fine with this idea.

>>  Also restricting a privileged user from ioctls with this
>> whitelist approach is going to be pointless because, if the whitelist is
>> configurable from userspace, they will just be able to modify the
>> whitelist.
>>
>>>
>>> The key point is to not only check for TIOCSTI, but instead check for
>>> a whitelisted ioctl.
>>>
>>> What would the whitelist look like?  Should configuing that be the way
>>> that you enable/disable, instead of the sysctl in this patchset?  So
>>> by default the whitelist includes all ioctls (no change), but things
>>> like sandboxes/sudo/container-starts can clear out the whitelist?
>>>
>>
>> I'm fine with moving this to an LSM that whitelists ioctls. I also want
> 
> Right -  what else would go into the whitelist?  may_mmap?
> 
>> to understand what a whitelist would like look and how you would
>> configure it? Does a sysctl that is a list of allowed ioctls work? I
>> don't want to just have a static whitelist that you can't change without
>> recompiling your kernel.
>>
>> just running a sysctl -a on a linux box shows me one thing that looks
>> like a list: net.core.flow_limit_cpu_bitmap