Re: [PATCH v5 next 1/5] modules:capabilities: add request_module_cap()

[Kees Cook]

On Tue, Nov 28, 2017 at 2:12 PM, Luis R. Rodriguez <mcgrof@xxxxxxxxxx> 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 <mcgrof@xxxxxxxxxx> 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

-- 
Kees Cook
Pixel Security