Date: Tue, 28 Nov 2017 14:18:18 -0800 From: Kees Cook <keescook@...omium.org> To: "Luis R. Rodriguez" <mcgrof@...nel.org> Cc: 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 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(). > 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
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