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Date: Tue, 28 Nov 2017 14:18:18 -0800
From: Kees Cook <>
To: "Luis R. Rodriguez" <>
Cc: Djalal Harouni <>, 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 2:12 PM, Luis R. Rodriguez <> wrote:
> On Tue, Nov 28, 2017 at 01:39:58PM -0800, Kees Cook wrote:
>> On Tue, Nov 28, 2017 at 1:16 PM, Luis R. Rodriguez <> wrote:
>> > And *all* auto-loading uses aliases? What's the difference between auto-loading
>> > and direct-loading?
>> The difference is the process privileges. Unprivilged autoloading
>> (e.g. int n_hdlc = N_HDLC; ioctl(fd,
>> TIOCSETD, &n_hdlc)), triggers a privileged call to finit_module()
>> under CAP_SYS_MODULE.
> Ah, so system call implicated request_module() calls.

Yup. Unprivileged user does something that ultimately hits a
request_module() in the kernel. Then the kernel calls out with the
usermode helper (which has CAP_SYS_MODULE) and calls finit_module().

> OK and since CAP_SYS_MODULE is much more restrictive one could argue, what's the
> point here?

The goal is to block an unprivileged user from being able to trigger a
module load without blocking root from loading modules directly.


Kees Cook
Pixel Security

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