Re: [PATCH v3 0/7] User namespace mount updates

Austin S Hemmelgarn
2015-11-18T15:38:58Z

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On 2015-11-18 09:30, Seth Forshee wrote:
> On Wed, Nov 18, 2015 at 07:46:53AM -0500, Austin S Hemmelgarn wrote:
>> On 2015-11-17 17:01, Seth Forshee wrote:
>>> On Tue, Nov 17, 2015 at 09:05:42PM +0000, Al Viro wrote:
>>>> On Tue, Nov 17, 2015 at 03:39:16PM -0500, Austin S Hemmelgarn wrote:
>>>>
>>>>>> This is absolutely insane, no matter how much LSM snake oil you slatter on
>>>>>> the whole thing. All of a sudden you are exposing a huge attack surface
>>>>>> in the place where it would hurt most and as the consolation we are offered
>>>>>> basically "Ted is willing to fix holes when they are found".
>>>
>>> None of the LSM changes are intended to protect against attacks from
>>> these sorts of attacks at all, so that's irrelevant.
>>>
>>> As I said before, I'm also working to find holes up front. That plus a
>>> commitment from the maintainer seems like a good start at least. What
>>> bar would you set for a given filesystem to be considered "safe enough"?
>>>
>>>>> For the context of static image attacks, anything that's foun
>>>>> _needs_ to be fixed regardless, and unless you can find some way to
>>>>> actually prevent attacks on mounted filesystems that doesn't involve
>>>>> a complete re-write of the filesystem drivers, then there's not much
>>>>> we can do about it. Yes, unprivileged mounts expose an attack
>>>>> surface, but so does userspace access to the network stack, and so
>>>>> do a lot of other features that are considered essential in a modern
>>>>> general purpose operating system.
>>>>
>>>> "X is exposes an attack surface. Y exposes a diferent attack surface.
>>>> Y is considered important. Therefore X is important enough to implement it"
>>>>
>>>> Right...
>>>
>>> That isn't the argument he made. I would summarize the argument as,
>>> "Saying that X exposes an attack surface isn't by itself enough to
>>> reject X, otherwise we wouldn't expose anything (such as example Y)."
>> It's good to see someone understood my meaning...
>>>
>>> You believe that the attack surface is too large, and that's
>>> understandable. Is it your opinion that this is a fundamental problem
>>> for an in-kernel filesystem driver, i.e. that we can never be confident
>>> enough in an in-kernel filesystem parser to allow untrusted data? If
>>> not, what would it take to establish a level of confidence that you
>>> would be comfortable with?
>> While I can't speak for Al's opinion on this, I would like to point
>> out my earlier comment:
>>> It's unfeasible from a practical standpoint to expect filesystems
>> to > assume that stuff they write might change under them due to
>> malicious > intent of a third party.
>
> So maybe the first requirement is that the user cannot modify the
> backing store directly while the device is mounted.
>
>> We can't protect against everything, not without making the system
>> completely unusable for general purpose computing. There is always
>> some degree of trust involved in usage of a computer, the OS has to
>> trust that the hardware works correctly, the administrator has to
>> trust the OS to behave correctly, and the users have to trust the
>> administrator. The administrator also needs to have at least some
>> trust in the users, otherwise he shouldn't be allowing them to use
>> the system.
>>
>> Perhaps we should have an option that can only be enabled on
>> creation of the userns that would allow it to use regular kernel
>> mounts, and without that option we default to only allowing FUSE and
>> a couple of virtual filesystems (like /proc and devtmpfs).
>
> I've considered the idea of something more global like a sysctl, or a
> per-filesystem knob in sysfs. I guess a per-container knob is another
> option, I'm not sure what interface we use to expose it though.
>
The most useful way I can see of implementing this would be to have an
option on container creation that controls whether kernel mounts are
allowed or not (possibly have it allow any of {no mounts, only FUSE
mounts, all mounts}), and then have a sysctl to set the default for
containers created without this option (and possibly one to force all
containers to ignore the option, and just use the default).

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