Date: Tue, 26 Jan 2016 09:56:26 -0500 From: Josh Boyer <jwboyer@...oraproject.org> To: "Austin S. Hemmelgarn" <ahferroin7@...il.com> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Kees Cook <keescook@...omium.org>, Andy Lutomirski <luto@...capital.net>, Andrew Morton <akpm@...ux-foundation.org>, Al Viro <viro@...iv.linux.org.uk>, Richard Weinberger <richard@....at>, Robert Święcki <robert@...ecki.net>, Dmitry Vyukov <dvyukov@...gle.com>, David Howells <dhowells@...hat.com>, Miklos Szeredi <mszeredi@...e.cz>, Kostya Serebryany <kcc@...gle.com>, Alexander Potapenko <glider@...gle.com>, Eric Dumazet <edumazet@...gle.com>, Sasha Levin <sasha.levin@...cle.com>, "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [PATCH 0/2] sysctl: allow CLONE_NEWUSER to be disabled On Tue, Jan 26, 2016 at 9:46 AM, Austin S. Hemmelgarn <ahferroin7@...il.com> wrote: > On 2016-01-26 09:38, Josh Boyer wrote: >> >> On Mon, Jan 25, 2016 at 11:57 PM, Eric W. Biederman >> <ebiederm@...ssion.com> wrote: >>> >>> Kees Cook <keescook@...omium.org> writes: >>> >>>> On Mon, Jan 25, 2016 at 11:33 AM, Eric W. Biederman >>>> <ebiederm@...ssion.com> wrote: >>>>> >>>>> Kees Cook <keescook@...omium.org> writes: >>>>>> >>>>>> >>>>>> Well, I don't know about less weird, but it would leave a unneeded >>>>>> hole in the permission checks. >>>>> >>>>> >>>>> To be clear the current patch has my: >>>>> >>>>> Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com> >>>>> >>>>> The code is buggy, and poorly thought through. Your lack of interest >>>>> in >>>>> fixing the bugs in your patch is distressing. >>>> >>>> >>>> I'm not sure where you see me having a "lack of interest". The >>>> existing cap-checking sysctls have a corner-case bug, which is >>>> orthogonal to this change. >>> >>> >>> That certainly doesn't sound like you have any plans to change anything >>> there. >>> >>>>> So broken code, not willing to fix. No. We are not merging this >>>>> sysctl. >>>> >>>> >>>> I think you're jumping to conclusions. :) >>> >>> >>> I think I am the maintainer. >>> >>> What you are proposing is very much something that is only of interst to >>> people who are not using user namespaces. It is fatally flawed as >>> a way to avoid new attack surfaces for people who don't care as the >>> sysctl leaves user namespaces enabled by default. It is fatally flawed >>> as remediation to recommend to people to change if a new user namespace >>> related but is discovered. Any running process that happens to be >>> created while user namespace creation was enabled will continue to >>> exist. Effectively a reboot will be required as part of a mitigation. >>> Many sysadmins will get that wrong. >>> >>> I can't possibly see your sysctl as proposed achieving it's goals. A >>> person has to be entirely too aware of subtlety and nuance to use it >>> effectively. >> >> >> What you're saying is true for the "oh crap" case of a new userns >> related CVE being found. However, there is the case where sysadmins >> know for a fact that a set of machines should not allow user >> namespaces to be enabled. Currently they have 2 choices, 1) use their >> distro kernel as-is, which may not meet their goal of having userns >> disabled, or 2) rebuild their kernel to disable it, which may >> invalidate any support contracts they have. >> >> I tend to agree with you on the lack of value around runtime >> mitigation, but allowing an admin to toggle this as a blatant on/off >> switch on reboot does have value. >> >>>> This feature is already implemented by two distros, and likely wanted >>>> by others. We cannot ignore that. The sysctl default doesn't change >>>> the existing behavior, so this doesn't get in your way at all. Can you >>>> please respond to my earlier email where I rebutted each of your >>>> arguments against it? Just saying "no" and putting words in my mouth >>>> isn't very productive. >>> >>> >>> Calling people who make mistakes insane is not a rebuttal. In security >>> usability matters, and your sysctl has low usability. >>> >>> Further you seem to have missed something crucial in your understanding. >>> As was explained earlier the sysctl was added to ubuntu to allow early >>> adopters to experiment not as a long term way of managing user >>> namespaces. >>> >>> >>> What sounds like a generally useful feature that would cover your use >>> case and many others is a per user limit on the number of user >>> namespaces users may create. >> >> >> Where that number may be zero? I don't see how that is really any >> better than a sysctl. Could you elaborate? > > It's a better option because it would allow better configurability. Take for > example a single user desktop system with some network daemons. On such a > system, the actual login used for the graphical environment by the user > should be allowed at least a few user namespaces, because some software > depends on them for security (Chrome for example, as well as some distro's > build systems), but system users should be limited to at most one if they > need it, and ideally zero, so that remote exploits couldn't give access to a > user namespace. > > Conversely, on a server system, it's not unreasonable to completely disable > user namespaces for almost everything, except for giving one to services > that use them properly for sand-boxing. OK, so better granularity. Fine. > I will state though that I only feel this is a better solution given that > two criteria are met: > 1. You can set 0 as the limit. > 2. You can configure this without needing some special software (this in > particular means that seccomp is not an option). I'd have to add 3. You can set a global default for all users that can be overridden on a per user basis. Otherwise you play whack-a-mole with every new user or daemon that adds its own uid. josh
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