On Mon, Aug 29, 2022 at 05:33:04PM +0200, Christian Brauner wrote: > On Fri, Aug 26, 2022 at 04:00:39PM -0500, Serge Hallyn wrote: > > On Fri, Aug 26, 2022 at 05:00:51PM +0000, Song Liu wrote: > > > > > > > > > > On Aug 26, 2022, at 8:24 AM, Serge E. Hallyn <serge@xxxxxxxxxx> wrote: > > > > > > > > On Thu, Aug 25, 2022 at 09:58:46PM +0000, Song Liu wrote: > > > >> > > > >> > > > >>> On Aug 25, 2022, at 12:19 PM, Paul Moore <paul@xxxxxxxxxxxxxx> wrote: > > > >>> > > > >>> On Thu, Aug 25, 2022 at 2:15 PM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > > > >>>> Paul Moore <paul@xxxxxxxxxxxxxx> writes: > > > >>>>> On Fri, Aug 19, 2022 at 10:45 AM Serge E. Hallyn <serge@xxxxxxxxxx> wrote: > > > >>>>>> I am hoping we can come up with > > > >>>>>> "something better" to address people's needs, make everyone happy, and > > > >>>>>> bring forth world peace. Which would stack just fine with what's here > > > >>>>>> for defense in depth. > > > >>>>>> > > > >>>>>> You may well not be interested in further work, and that's fine. I need > > > >>>>>> to set aside a few days to think on this. > > > >>>>> > > > >>>>> I'm happy to continue the discussion as long as it's constructive; I > > > >>>>> think we all are. My gut feeling is that Frederick's approach falls > > > >>>>> closest to the sweet spot of "workable without being overly offensive" > > > >>>>> (*cough*), but if you've got an additional approach in mind, or an > > > >>>>> alternative approach that solves the same use case problems, I think > > > >>>>> we'd all love to hear about it. > > > >>>> > > > >>>> I would love to actually hear the problems people are trying to solve so > > > >>>> that we can have a sensible conversation about the trade offs. > > > >>> > > > >>> Here are several taken from the previous threads, it's surely not a > > > >>> complete list, but it should give you a good idea: > > > >>> > > > >>> https://lore.kernel.org/linux-security-module/CAHC9VhQnPAsmjmKo-e84XDJ1wmaOFkTKPjjztsOa9Yrq+AeAQA@xxxxxxxxxxxxxx/ > > > >>> > > > >>>> As best I can tell without more information people want to use > > > >>>> the creation of a user namespace as a signal that the code is > > > >>>> attempting an exploit. > > > >>> > > > >>> Some use cases are like that, there are several other use cases that > > > >>> go beyond this; see all of our previous discussions on this > > > >>> topic/patchset. As has been mentioned before, there are use cases > > > >>> that require improved observability, access control, or both. > > > >>> > > > >>>> As such let me propose instead of returning an error code which will let > > > >>>> the exploit continue, have the security hook return a bool. With true > > > >>>> meaning the code can continue and on false it will trigger using SIGSYS > > > >>>> to terminate the program like seccomp does. > > > >>> > > > >>> Having the kernel forcibly exit the process isn't something that most > > > >>> LSMs would likely want. I suppose we could modify the hook/caller so > > > >>> that *if* an LSM wanted to return SIGSYS the system would kill the > > > >>> process, but I would want that to be something in addition to > > > >>> returning an error code like LSMs normally do (e.g. EACCES). > > > >> > > > >> I am new to user_namespace and security work, so please pardon me if > > > >> anything below is very wrong. > > > >> > > > >> IIUC, user_namespace is a tool that enables trusted userspace code to > > > >> control the behavior of untrusted (or less trusted) userspace code. > > > > > > > > No. user namespaces are not a way for more trusted code to control the > > > > behavior of less trusted code. > > > > > > Hmm.. In this case, I think I really need to learn more. > > > > > > Thanks for pointing out my misunderstanding. > > > > (I thought maybe Eric would chime in with a better explanation, but I'll > > fill it in for now :) > > > > One of the main goals of user namespaces is to allow unprivileged users > > to do things like chroot and mount, which are very useful development > > tools, without needing admin privileges. So it's almost the opposite > > of what you said: rather than to enable trusted userspace code to control > > the behavior of less trusted code, it's to allow less privileged code to > > do things which do not affect other users, without having to assume *more* > > privilege. > > > > To be precise, the goals were: > > > > 1. uid mapping - allow two users to both "use uid 500" without conflicting > > 2. provide (unprivileged) users privilege over their own resources > > 3. absolutely no extra privilege over other resources > > 4. be able to nest > > > > While (3) was technically achieved, the problem we have is that > > (2) provides unprivileged users the ability to exercise kernel code > > which they previously could not. > > The consequence of the refusal to give users any way to control whether > or not user namespaces are available to unprivileged users is that a > non-significant number of distros still carry the same patch for about > 10 years now that adds an unprivileged_userns_clone sysctl to restrict > them to privileged users. That includes current Debian and Archlinux btw. Hi Christian, I'm wondering about your placement of this argument in the thread, and whether you interpreted what I said above as an argument against this patchset, or whether you're just expanding on what I said. > The LSM hook is a simple way to allow administrators to control this and (I think the "control" here is suboptimal, but I've not seen - nor conceived of - anything better as of yet) > will allow user namespaces to be enabled in scenarios where they > would otherwise not be accepted precisely because they are available to > unprivileged users. > > I fully understand the motivation and usefulness in unprivileged > scenarios but it's an unfounded fear that giving users the ability to > control user namespace creation via an LSM hook will cause proliferation > of setuid binaries (Ignoring for a moment that any fully unprivileged > container with useful idmappings has to rely on the new{g,u}idmap setuid > binaries to setup useful mappings anyway.) or decrease system safety let > alone cause regressions (Which I don't think is an applicable term here > at all.). Distros that have unprivileged user namespaces turned on by > default are extremely unlikely to switch to an LSM profile that turns > them off and distros that already turn them off will continue to turn > them off whether or not that LSM hook is available. > > It's much more likely that workloads that want to minimize their attack > surface while still getting the benefits of user namespaces for e.g. > service isolation will feel comfortable enabling them for the first time > since they can control them via an LSM profile.
