On Thu, Oct 22, 2015 at 1:45 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > > Thank you for a creative solution to a problem that you perceive. I > appreciate it when people aim to solve problems they see. > > Tobias Markus <tobias@xxxxxxxxx> writes: > >> On 17.10.2015 23:55, Serge E. Hallyn wrote: >>> On Sat, Oct 17, 2015 at 05:58:04PM +0200, Tobias Markus wrote: >>>> Add capability CAP_SYS_USER_NS. >>>> Tasks having CAP_SYS_USER_NS are allowed to create a new user namespace >>>> when calling clone or unshare with CLONE_NEWUSER. >>>> >>>> Rationale: >>>> >>>> Linux 3.8 saw the introduction of unpriviledged user namespaces, >>>> allowing unpriviledged users (without CAP_SYS_ADMIN) to be a "fake" root >>>> inside a separate user namespace. Before that, any namespace creation >>>> required CAP_SYS_ADMIN (or, in practice, the user had to be root). >>>> Unfortunately, there have been some security-relevant bugs in the >>>> meantime. Because of the fairly complex nature of user namespaces, it is >>>> reasonable to say that future vulnerabilties can not be excluded. Some >>>> distributions even wholly disable user namespaces because of this. >>> >>> Fwiw I'm not in favor of this. Debian has a patch (I believe the one >>> I originally wrote for Ubuntu but which Ubuntu dropped long ago) adding a >>> sysctl, off by default, for enabling user namespaces. >> >> While it certainly works, enabling a feature like this at runtime >> doesn't seem like a long term solution. >> >> The fact that Debian added this patch in the first place already >> demonstrates that there is demand for a way to limit unpriviledged user >> namespace creation. Please, don't get me wrong: I would *really like* to >> see widespread adoption and continued development of user namespaces! >> But the status quo remains: Distributions outright disabling user >> namespaces (e.g. Arch Linux) won't make it easier. > > Let me say I applaud Arch Linux for not doing what so many distributions > do and enable every feature in the kernel. I appreciate a distribution > that does not enable interesting kernel features while they are still > having their bugs shaken out of them. > > I also think Debians approach to limit things while they mature is also > wisdom. > >>> Posix capabilities are intended for privileged actions, not for >>> actions which explicitly should not require privilege, but which >>> we feel are in development. >>> >> >> Certainly, in an ideal world, user namespaces will never lead to any >> kernel-level exploits. But reality is different: There *have been* >> serious kernel vulnerabilities due to user namespaces, and there *will >> be* serious kernel vulnerabilities due to user namespaces. > > When you start talk about the future that is not yet real you have > stopped talking about reality. That sounds like a pessimists world view > rather than reality. > > The reality is new features are buggy and take time to mature. It takes > time for understanding to percolate through peoples heads. > >> Now, those are the alternatives imho: >> >> * Status quo: Some distributions will disable user namespaces by default >> in some way or another. User wishing to use user namespaces will have to >> use a custom kernel or enable a sysctl flag that was patched in by the >> downstream developers. On distributions that enable user namespaces by >> default, even users that don't wish to use them in the first places will >> be affected by vulnerabilities. > > Again I disagree. I see distributions waiting to enable user namespaces > until they mature and until they are interesting enough. I do not see > rushing to enable the newest features as wisdom, unless that the point > of your distribution is to enable people to play with the latest > features. > > I suspect we are quickly coming to a point where user namespaces will be > sufficiently compelling that they will be enabled more widely. > > > At this point the most helpful things I can see to be done are. > - Verify all userns related fixes have made it back into 4.1.x > - Play with and/or audit the userns code to see if more bugs can be > found. > - Analyze user namespaces and see if they are uniquely worse than > anything else. > > I agree that if user namespaces pose a unique security challenge to > the kernel we should do something about them. I think it is a healthy > question to ask. For the conversation to be productive I think we need > numbers and analsysis, not just worst case analsysis based on fear. To > date all I see are teething pains. > > My back of the napkin analysis is that there are maybe 3,000 lines of > code executed in user namespaces (mostly from fs/namespace.c) that > are not otherwise reachable from unprivileged users, while there are > perhaps 100,000 - 250,000 lines of code reachable by unprivileged users > (not counting drivers). At the risk of pointing out a can of worms, the attack surface also includes things like the iptables configuration APIs, parsers, and filter/conntrack/action modules. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
