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Message-ID: <CAGXu5jJTdL7To2JsQUXyg6B7Xbb0kow6sXH-L+teVi88=gm7MQ@mail.gmail.com> Date: Fri, 21 Apr 2017 16:40:37 -0700 From: Kees Cook <keescook@...omium.org> To: Andy Lutomirski <luto@...capital.net> Cc: Andy Lutomirski <luto@...nel.org>, Djalal Harouni <tixxdz@...il.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, "Serge E. Hallyn" <serge@...lyn.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LSM List <linux-security-module@...r.kernel.org>, Linux API <linux-api@...r.kernel.org>, Dongsu Park <dpark@...teo.net>, Casey Schaufler <casey@...aufler-ca.com>, James Morris <james.l.morris@...cle.com>, Paul Moore <paul@...l-moore.com>, Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Jonathan Corbet <corbet@....net>, Jessica Yu <jeyu@...hat.com>, Rusty Russell <rusty@...tcorp.com.au>, Arnaldo Carvalho de Melo <acme@...hat.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Ingo Molnar <mingo@...nel.org>, belakhdar abdeldjalil <zendyani@...il.com>, Peter Zijlstra <peterz@...radead.org> Subject: Re: [PATCH v3 2/2] modules:capabilities: add a per-task modules autoload restriction On Fri, Apr 21, 2017 at 4:28 PM, Andy Lutomirski <luto@...capital.net> wrote: > On Fri, Apr 21, 2017 at 4:19 PM, Kees Cook <keescook@...omium.org> wrote: >> On Wed, Apr 19, 2017 at 7:41 PM, Andy Lutomirski <luto@...nel.org> wrote: >>> On Wed, Apr 19, 2017 at 4:43 PM, Kees Cook <keescook@...omium.org> wrote: >>>> On Wed, Apr 19, 2017 at 4:15 PM, Andy Lutomirski <luto@...nel.org> wrote: >>>>> On Wed, Apr 19, 2017 at 3:20 PM, Djalal Harouni <tixxdz@...il.com> wrote: >>>>>> +/* Sets task's modules_autoload */ >>>>>> +static inline int task_set_modules_autoload(struct task_struct *task, >>>>>> + unsigned long value) >>>>>> +{ >>>>>> + if (value > MODULES_AUTOLOAD_DISABLED) >>>>>> + return -EINVAL; >>>>>> + else if (task->modules_autoload > value) >>>>>> + return -EPERM; >>>>>> + else if (task->modules_autoload < value) >>>>>> + task->modules_autoload = value; >>>>>> + >>>>>> + return 0; >>>>>> +} >>>>> >>>>> This needs to be more locked down. Otherwise someone could set this >>>>> and then run a setuid program. Admittedly, it would be quite odd if >>>>> this particular thing causes a problem, but the issue exists >>>>> nonetheless. >>>> >>>> Eeeh, I don't agree this needs to be changed. APIs provided by modules >>>> are different than the existing privilege-manipulation syscalls this >>>> concern stems from. Applications are already forced to deal with >>>> things being missing like this in the face of it simply not being >>>> built into the kernel. >>>> >>>> Having to hide this behind nnp seems like it'd reduce its utility... >>>> >>> >>> I think that adding an inherited boolean to task_struct that can be >>> set by unprivileged tasks and passed to privileged tasks is a terrible >>> precedent. Ideally someone would try to find all the existing things >>> like this and kill them off. >> >> (Tristate, not boolean, but yeah.) >> >> I see two others besides seccomp and nnp: >> >> PR_MCE_KILL > > Well, that's interesting. That should presumably be reset on setuid > exec or something. > >> PR_SET_THP_DISABLE > > Um. At least that's just a performance issue. > >> >> I really don't think this needs nnp protection. >> >>> I agree that I don't see how one would exploit this particular >>> feature, but I still think I dislike the approach. This is a slippery >>> slope to adding a boolean for perf_event_open(), unshare(), etc, and >>> we should solve these for real rather than half-arsing them IMO. >> >> I disagree (obviously); this would be protecting the entire module >> autoload attack surface. That's hardly a specific control, and it's a >> demonstrably needed flag. >> > > The list is just going to get longer. We should probably have controls for: > > - Use of perf. Unclear how fine grained they should be. This can already be "given up" by a process by using seccomp. The system-wide setting is what's missing here, and that's a whole other thread already even though basically every distro has implemented the = 3 sysctl knob level. > - Creation of new user namespaces. Possibly also use of things like > iptables without global privilege. This is another one that can be controlled by seccomp. The system-wide setting already exists in /proc/sys/user/max_user_namespaces. > - Ability to look up tasks owned by different uids (or maybe other > tasks *at all*) by pid/tid. Conceptually, this is easy. The API is > the only hard part, I think. The attack surface here is relatively small compared to the other examples. > - Ability to bind ports, maybe? seccomp and maybe a sysctl? I'd have to look at that more carefully, but again, this isn't a comparable attack-surface/confinement issue. > My point is that all of these need some way to handle configuration > and inheritance, and I don't think that a bunch of per-task prctls is > the right way. As just an example, saying that interactive users can > autoload modules but other users can't, or that certain systemd > services can, etc, might be nice. Linus already complained that he > (i.e. user "torvalds" or whatever) should be able to profile the > kernel but that other uids should not be able to. > > I personally like my implicit_rights idea, and it might be interesting > to prototype it. I don't like blocking a needed feature behind a large super-feature that doesn't exist yet. We'd be able to refactor this code into using such a thing in the future, so I'd prefer to move ahead with this since it would stop actual exploits. -Kees -- Kees Cook Pixel Security
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