On Mon, May 22, 2017 at 12:55 PM, Djalal Harouni [off-list ref] wrote:

On Mon, May 22, 2017 at 6:43 PM, Solar Designer [off-list ref] wrote:

quoted

On Mon, May 22, 2017 at 03:49:15PM +0200, Djalal Harouni wrote:

quoted

On Mon, May 22, 2017 at 2:08 PM, Solar Designer [off-list ref] wrote:

quoted

On Mon, May 22, 2017 at 01:57:03PM +0200, Djalal Harouni wrote:

quoted

*) 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