Date: Tue, 28 Nov 2017 23:48:49 +0100 From: "Luis R. Rodriguez" <mcgrof@...nel.org> To: Kees Cook <keescook@...omium.org> Cc: "Luis R. Rodriguez" <mcgrof@...nel.org>, Djalal Harouni <tixxdz@...il.com>, Andy Lutomirski <luto@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, James Morris <james.l.morris@...cle.com>, Ben Hutchings <ben.hutchings@...ethink.co.uk>, Solar Designer <solar@...nwall.com>, Serge Hallyn <serge@...lyn.com>, Jessica Yu <jeyu@...nel.org>, Rusty Russell <rusty@...tcorp.com.au>, LKML <linux-kernel@...r.kernel.org>, linux-security-module <linux-security-module@...r.kernel.org>, kernel-hardening@...ts.openwall.com, Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...nel.org>, "David S. Miller" <davem@...emloft.net>, Network Development <netdev@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [PATCH v5 next 1/5] modules:capabilities: add request_module_cap() On Tue, Nov 28, 2017 at 02:18:18PM -0800, Kees Cook wrote: > On Tue, Nov 28, 2017 at 2:12 PM, Luis R. Rodriguez <mcgrof@...nel.org> 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@...nel.org> 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(). Thanks, using this terminology is much better to understand than auto-loading, given it does make it clear an unprivileged call was one that initiated the request_module() call, there are many uses of request_module() which *are* privileged. > > 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. I see now. Do we have an audit of all system calls which implicate a request_module() call? Networking is a good example for sure to start off with but I was curious if we have a grasp of how wide spread this could be. I'll go review the patches again now with all this in mind. Luis
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