Date: Mon, 22 May 2017 16:07:56 -0700 From: Kees Cook <keescook@...omium.org> To: Djalal Harouni <tixxdz@...il.com> Cc: Solar Designer <solar@...nwall.com>, linux-kernel <linux-kernel@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, LSM List <linux-security-module@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, Andy Lutomirski <luto@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, Rusty Russell <rusty@...tcorp.com.au>, "Serge E. Hallyn" <serge@...lyn.com>, Jessica Yu <jeyu@...hat.com>, "David S. Miller" <davem@...emloft.net>, James Morris <james.l.morris@...cle.com>, Paul Moore <paul@...l-moore.com>, Stephen Smalley <sds@...ho.nsa.gov>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, Ingo Molnar <mingo@...nel.org>, Linux API <linux-api@...r.kernel.org>, Dongsu Park <dpark@...teo.net>, Casey Schaufler <casey@...aufler-ca.com>, Jonathan Corbet <corbet@....net>, Arnaldo Carvalho de Melo <acme@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Zendyani <zendyani@...il.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, Al Viro <viro@...iv.linux.org.uk>, Ben Hutchings <ben.hutchings@...ethink.co.uk> Subject: Re: [PATCH v4 next 0/3] modules: automatic module loading restrictions On Mon, May 22, 2017 at 12:55 PM, Djalal Harouni <tixxdz@...il.com> wrote: > On Mon, May 22, 2017 at 6:43 PM, Solar Designer <solar@...nwall.com> wrote: >> On Mon, May 22, 2017 at 03:49:15PM +0200, Djalal Harouni wrote: >>> On Mon, May 22, 2017 at 2:08 PM, Solar Designer <solar@...nwall.com> wrote: >>> > On Mon, May 22, 2017 at 01:57:03PM +0200, Djalal Harouni wrote: >>> >> *) When modules_autoload_mode is set to (2), automatic module loading is >>> >> disabled for all. Once set, this value can not be changed. >>> > >>> > What purpose does this securelevel-like property ("Once set, this value >>> > can not be changed.") serve here? I think this mode 2 is needed, but >>> > without this extra property, which is bypassable by e.g. explicitly >>> > loaded kernel modules anyway (and that's OK). >>> >>> My reasoning about "Once set, this value can not be changed" is mainly for: >>> >>> If you have some systems where modules are not updated for any given >>> reason, then the only one who will be able to load a module is an >>> administrator, basically this is a shortcut for: >>> >>> * Apps/services can run with CAP_NET_ADMIN but they are not allowed to >>> auto-load 'netdev' modules. >>> >>> * Explicitly loading modules can be guarded by seccomp filters *per* >>> app, so even if these apps have >>> CAP_SYS_MODULE they won't be able to explicitly load modules, one >>> has to remount some sysctl /proc/ entries read-only here and remove >>> CAP_SYS_ADMIN for all apps anyway. >>> >>> This mainly serves the purpose of these systems that do not receive >>> updates, if I don't want to expose those kernel interfaces what should >>> I do ? then if I want to unload old versions and replace them with new >>> ones what operation should be allowed ? and only real root of the >>> system can do it. Hence, the "Once set, this value can not be changed" >>> is more of a shortcut, also the idea was put in my mind based on how >>> "modules_disabled" is disabled forever, and some other interfaces. I >>> would say: it is easy to handle a transition from 1) "hey this system >>> is still up to date, some features should be exposed" to 2) "this >>> system is not up to date anymore, only root should expose some >>> features..." >>> >>> Hmm, I am not sure if this answers your question ? :-) >> >> This answers my question, but in a way that I summarize as "there's no >> good reason to include this securelevel-like property". >> > > Hmm, sorry I did forget to add in my previous comment that with such > systems, CAP_SYS_MODULE can be used to reset the > "modules_autoload_mode" sysctl back from mode 2 to mode 1, even if we > disable it privileged tasks can be triggered to overwrite the sysctl > flag and get it back unless /proc is read-only... that's one of the > points, it should not be so easy to relax it. I'm on the fence. For modules_disabled and Yama, it was tied to CAP_SYS_ADMIN, basically designed to be a at-boot setting that could not later be undone by an attacker gaining that privilege, keeping them out of either kernel memory or existing user process memory. Here, it's CAP_SYS_MODULE... it's hard to imagine the situation where a CAP_SYS_MODULE-capable process could write to this sysctl but NOT issue direct modprobe requests, but it's _possible_ via crazy symlink games to trick capable processes into writing to sysctls. We've seen this multiple times before, and it's a way for attackers to turn a single privileged write into a privileged exec. I might turn the question around, though: why would we want to have it changeable at this setting? I'm fine leaving that piece off, either way. -Kees -- Kees Cook Pixel Security
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.