Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Tue, 28 Nov 2017 22:33:27 +0100
From: Djalal Harouni <>
To: "Luis R. Rodriguez" <>
Cc: Kees Cook <>, Andy Lutomirski <>, 
	Andrew Morton <>, James Morris <>, 
	Ben Hutchings <>, Solar Designer <>, 
	Serge Hallyn <>, Jessica Yu <>, 
	Rusty Russell <>, LKML <>, 
	linux-security-module <>,, 
	Jonathan Corbet <>, Ingo Molnar <>, 
	"David S. Miller" <>, Network Development <>, 
	Peter Zijlstra <>, Linus Torvalds <>
Subject: Re: [PATCH v5 next 1/5] modules:capabilities: add request_module_cap()

On Tue, Nov 28, 2017 at 10:16 PM, Luis R. Rodriguez <> wrote:
> On Tue, Nov 28, 2017 at 12:11:34PM -0800, Kees Cook wrote:
>> On Tue, Nov 28, 2017 at 11:14 AM, Luis R. Rodriguez <> wrote:
>> > kmod is just a helper to poke userpsace to load a module, that's it.
>> >
>> > The old init_module() and newer finit_module() do the real handy work or
>> > module loading, and both currently only use may_init_module():
>> >
>> > static int may_init_module(void)
>> > {
>> >         if (!capable(CAP_SYS_MODULE) || modules_disabled)
>> >                 return -EPERM;
>> >
>> >         return 0;
>> > }
>> >
>> > This begs the question:
>> >
>> >   o If userspace just tries to just use raw finit_module() do we want similar
>> >     checks?
>> >
>> > Otherwise, correct me if I'm wrong this all seems pointless.
>> Hm? That's direct-loading, not auto-loading. This series is only about
>> auto-loading.
> And *all* auto-loading uses aliases? What's the difference between auto-loading
> and direct-loading?

Not all auto-loading uses aliases, auto-loading is when kernel code
calls request_module() to loads the feature that was not present, and
direct-loading in this thread is the direct syscalls like

>> We already have a global sysctl for blocking direct-loading (modules_disabled).
> My point was that even if you have a CAP_NET_ADMIN check on request_module(),
> finit_module() will not check for it, so a crafty userspace could still try
> to just finit_module() directly, and completely then bypass the CAP_NET_ADMIN
> check.

The finit_module() uses CAP_SYS_MODULE which should allow all modules
and in this context it should be more privileged than CAP_NET_ADMIN
which is only for "netdev-%s" (to not load arbitrary modules with it).

finit_module() coming from request_module() always has the
CAP_NET_ADMIN, hence the check is done before.

> So unless I'm missing something, I see no point in adding extra checks for
> request_module() but nothing for the respective load_module().

I see, request_module() is called from kernel context which runs in
init namespace will full capabilities, the spawned userspace modprobe
will get CAP_SYS_MODULE and all other caps, then after comes modprobe
and load_module().

Btw as suggested by Linus I will update with request_module_cap() and
I can offer my help maintaining these bits too.

>   Luis


Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.