[kernel-hardening] [PATCH v4 next 0/3] modules: automatic module loading restrictions

[Kees Cook]

On Mon, May 22, 2017 at 12:55 PM, Djalal Harouni <tixxdz at gmail.com> wrote:
> On Mon, May 22, 2017 at 6:43 PM, Solar Designer <solar at openwall.com> wrote:
>> On Mon, May 22, 2017 at 03:49:15PM +0200, Djalal Harouni wrote:
>>> On Mon, May 22, 2017 at 2:08 PM, Solar Designer <solar at openwall.com> wrote:
>>> > On Mon, May 22, 2017 at 01:57:03PM +0200, Djalal Harouni wrote:
>>> >> *) When modules_autoload_mode is set to (2), automatic module loading is
>>> >> disabled for all. Once set, this value can not be changed.
>>> >
>>> > What purpose does this securelevel-like property ("Once set, this value
>>> > can not be changed.") serve here?  I think this mode 2 is needed, but
>>> > without this extra property, which is bypassable by e.g. explicitly
>>> > loaded kernel modules anyway (and that's OK).
>>>
>>> My reasoning about "Once set, this value can not be changed" is mainly for:
>>>
>>> If you have some systems where modules are not updated for any given
>>> reason, then the only one who will be able to load a module is an
>>> administrator, basically this is a shortcut for:
>>>
>>> * Apps/services can run with CAP_NET_ADMIN but they are not allowed to
>>> auto-load 'netdev' modules.
>>>
>>> * Explicitly loading modules can be guarded by seccomp filters *per*
>>> app, so even if these apps have
>>>   CAP_SYS_MODULE they won't be able to explicitly load modules, one
>>> has to remount some sysctl /proc/ entries read-only here and remove
>>> CAP_SYS_ADMIN for all apps anyway.
>>>
>>> This mainly serves the purpose of these systems that do not receive
>>> updates, if I don't want to expose those kernel interfaces what should
>>> I do ? then if I want to unload old versions and replace them with new
>>> ones what operation should be allowed ? and only real root of the
>>> system can do it. Hence, the "Once set, this value can not be changed"
>>> is more of a shortcut, also the idea was put in my mind based on how
>>> "modules_disabled" is disabled forever, and some other interfaces. I
>>> would say: it is easy to handle a transition from 1) "hey this system
>>> is still up to date, some features should be exposed" to 2) "this
>>> system is not up to date anymore, only root should expose some
>>> features..."
>>>
>>> Hmm, I am not sure if this answers your question ? :-)
>>
>> This answers my question, but in a way that I summarize as "there's no
>> good reason to include this securelevel-like property".
>>
>
> Hmm, sorry I did forget to add in my previous comment that with such
> systems, CAP_SYS_MODULE can be used to reset the
> "modules_autoload_mode" sysctl back from mode 2 to mode 1, even if we
> disable it privileged tasks can be triggered to overwrite the sysctl
> flag and get it back unless /proc is read-only... that's one of the
> points, it should not be so easy to relax it.

I'm on the fence. For modules_disabled and Yama, it was tied to
CAP_SYS_ADMIN, basically designed to be a at-boot setting that could
not later be undone by an attacker gaining that privilege, keeping
them out of either kernel memory or existing user process memory.
Here, it's CAP_SYS_MODULE... it's hard to imagine the situation where
a CAP_SYS_MODULE-capable process could write to this sysctl but NOT
issue direct modprobe requests, but it's _possible_ via crazy symlink
games to trick capable processes into writing to sysctls. We've seen
this multiple times before, and it's a way for attackers to turn a
single privileged write into a privileged exec.

I might turn the question around, though: why would we want to have it
changeable at this setting?

I'm fine leaving that piece off, either way.

-Kees

-- 
Kees Cook
Pixel Security
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