Date: Fri, 10 Nov 2017 13:30:56 +0900 From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com> To: "Eric W. Biederman" <ebiederm@...ssion.com> Cc: "Serge E. Hallyn" <serge@...lyn.com>, Christian Brauner <christian.brauner@...onical.com>, Boris Lukashev <blukashev@...pervictus.com>, Daniel Micay <danielmicay@...il.com>, Mahesh Bandewar <mahesh@...dewar.net>, LKML <linux-kernel@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Kernel-hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, Kees Cook <keescook@...omium.org>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net> Subject: Re: Re: [PATCH resend 2/2] userns: control capabilities of some user namespaces On Fri, Nov 10, 2017 at 6:58 AM, Eric W. Biederman <ebiederm@...ssion.com> wrote: > "Mahesh Bandewar (महेश बंडेवार)" <maheshb@...gle.com> writes: > >> [resend response as earlier one failed because of formatting issues] >> >> On Thu, Nov 9, 2017 at 12:21 PM, Serge E. Hallyn <serge@...lyn.com> wrote: >>> >>> On Thu, Nov 09, 2017 at 09:55:41AM +0900, Mahesh Bandewar (महेश बंडेवार) wrote: >>> > On Thu, Nov 9, 2017 at 4:02 AM, Christian Brauner >>> > <christian.brauner@...onical.com> wrote: >>> > > On Wed, Nov 08, 2017 at 03:09:59AM -0800, Mahesh Bandewar (महेश बंडेवार) wrote: >>> > >> Sorry folks I was traveling and seems like lot happened on this thread. :p >>> > >> >>> > >> I will try to response few of these comments selectively - >>> > >> >>> > >> > The thing that makes me hesitate with this set is that it is a >>> > >> > permanent new feature to address what (I hope) is a temporary >>> > >> > problem. >>> > >> I agree this is permanent new feature but it's not solving a temporary >>> > >> problem. It's impossible to assess what and when new vulnerability >>> > >> that could show up. I think Daniel summed it up appropriately in his >>> > >> response >>> > >> >>> > >> > Seems like there are two naive ways to do it, the first being to just >>> > >> > look at all code under ns_capable() plus code called from there. It >>> > >> > seems like looking at the result of that could be fruitful. >>> > >> This is really hard. The main issue that there were features designed >>> > >> and developed before user-ns days with an assumption that unprivileged >>> > >> users will never get certain capabilities which only root user gets. >>> > >> Now that is not true anymore with user-ns creation with mapping root >>> > >> for any process. Also at the same time blocking user-ns creation for >>> > >> eveyone is a big-hammer which is not needed too. So it's not that easy >>> > >> to just perform a code-walk-though and correct those decisions now. >>> > >> >>> > >> > It seems to me that the existing control in >>> > >> > /proc/sys/kernel/unprivileged_userns_clone might be the better duct tape >>> > >> > in that case. >>> > >> This solution is essentially blocking unprivileged users from using >>> > >> the user-namespaces entirely. This is not really a solution that can >>> > >> work. The solution that this patch-set adds allows unprivileged users >>> > >> to create user-namespaces. Actually the proposed solution is more >>> > >> fine-grained approach than the unprivileged_userns_clone solution >>> > >> since you can selectively block capabilities rather than completely >>> > >> blocking the functionality. >>> > > >>> > > I've been talking to Stéphane today about this and we should also keep in mind >>> > > that we have: >>> > > >>> > > chb@...ventiont|~ >>> > >> ls -al /proc/sys/user/ >>> > > total 0 >>> > > dr-xr-xr-x 1 root root 0 Nov 6 23:32 . >>> > > dr-xr-xr-x 1 root root 0 Nov 2 22:13 .. >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_cgroup_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_inotify_instances >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_inotify_watches >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_ipc_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_mnt_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_net_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_pid_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_user_namespaces >>> > > -rw-r--r-- 1 root root 0 Nov 8 19:48 max_uts_namespaces >>> > > >>> > > These files allow you to limit the number of namespaces that can be created >>> > > *per namespace* type. So let's say your system runs a bunch of user namespaces >>> > > you can do: >>> > > >>> > > chb@...ventiont|~ >>> > >> echo 0 > /proc/sys/user/max_user_namespaces >>> > > >>> > > So that the next time you try to create a user namespaces you'd see: >>> > > >>> > > chb@...ventiont|~ >>> > >> unshare -U >>> > > unshare: unshare failed: No space left on device >>> > > >>> > > So there's not even a need to upstream a new sysctl since we have ways of >>> > > blocking this. >>> > > >>> > I'm not sure how it's solving the problem that my patch-set is addressing? >>> > I agree though that the need for unprivileged_userns_clone sysctl goes >>> > away as this is equivalent to setting that sysctl to 0 as you have >>> > described above. >>> >>> oh right that was the reasoning iirc for not needing the other sysctl. >>> >>> > However as I mentioned earlier, blocking processes from creating >>> > user-namespaces is not the solution. Processes should be able to >>> > create namespaces as they are designed but at the same time we need to >>> > have controls to 'contain' them if a need arise. Setting max_no to 0 >>> > is not the solution that I'm looking for since it doesn't solve the >>> > problem. >>> >>> well yesterday we were told that was explicitly not the goal, but that was >>> not by you ... i just mention it to explain why we seem to be walking in >>> circles a bit. >>> >>> anyway the bounding set doesn't actually make sense so forget that. the >>> question then is just whether it makes sense to allow things to continue >>> at all in this situation. would you mind indulging me by giving one or two >>> concrete examples in the previous known cves of what capabilities you would >>> have dropped tto allow the rest to continue to be safely used? >>> >> Of course. Let's take an example of the CVE that I have mentioned in >> my cover-letter - >> CVE-2017-7308(https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-7308). >> It's well documented and even has a >> exploit(https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-7308) >> c-program that can demonstrate how it can be used against non-patched >> kernel. There is very nice blog >> post(https://googleprojectzero.blogspot.kr/2017/05/exploiting-linux-kernel-via-packet.html) >> about this vulnerability by Andrey Konovalov. >> >> This is about the AF_PACKET socket interface that is protected behind >> NET_RAW capability. This capability is not available to unprivileged >> user. However, any unprivileged user can get NET_RAW capability (as >> demonstrated in the cover-letter code that I have attached in this >> patch series) so this NET_RAW capability is available to any >> unprivileged user on the host if the kernel has user-namespaces >> available. >> >> With this patch-set applied, all that is needed is to flip a bit with >> the sysctl (kernel.controlled_userns_caps_whitelist) as demonstrated >> below - >> >> root@...h6:~# uname -a >> Linux lphh6 4.14.0-smp-DEV #97 SMP @1510203579 x86_64 GNU/Linux >> root@...h6:~# sysctl -q kernel.controlled_userns_caps_whitelist >> kernel.controlled_userns_caps_whitelist = 1f,ffffffff >> >> Now when I run the program (demo from the cover-letter) as a normal >> unprivileged user I can't create a RAW socket in init-ns but I can in >> the child-ns. >> >> dumbo@...h6:~$ /tmp/acquire_raw >> Attempting to open RAW socket before unshare()... >> socket() SOCK_RAW failed: : Operation not permitted >> Attempting to open RAW socket after unshare()... >> Successfully opened RAW-Sock after unshare(). >> dumbo@...h6:~$ >> >> Now as a root user. Take off CAP_NET_RAW >> >> root@...h6:~# sysctl -w kernel.controlled_userns_caps_whitelist=1f,ffffdfff >> kernel.controlled_userns_caps_whitelist = 1f,ffffdfff >> root@...h6:~# >> >> Now run the same program as an unprivileged user - >> >> dumbo@...h6:~$ /tmp/acquire_raw >> Attempting to open RAW socket before unshare()... >> socket() SOCK_RAW failed: : Operation not permitted >> Attempting to open RAW socket after unshare()... >> socket() SOCK_RAW failed: : Operation not permitted >> dumbo@...h6:~$ >> >> Notice that it has failed to create a raw socket in init and in child >> namespace. It's not blocking creation of user-namespaces but allowing >> admin turn individual capability bits on and off. >> >> This is very simplistic example of just demonstrating how capability >> bits turn-on/off works. So let's assume a sandboxed environment where >> we don't know what a binary that we are about run in an environment >> which is identified as susceptible. By turning off the NET_RAW bit, >> the admin gets an assurance that system is safe and if binary fails >> because it's not getting this capability then that bad but a sad >> consequence (without compromising the host integrity) but if it >> doesn't use the NET_RAW capability but any other combination of >> remaining 36 capabilities, it would get whatever is necessary. This >> means we can safely allow processes to create user-namespaces by >> taking off certain capabilities in question for temporary/extended >> period until proper fix is applied without compromising the system >> integrity. The impact will vary based on which capability is taken off >> and admin would / should be ware of for the environment that he/she is >> dealing with. > > My challenge with this reasoning is that I don't know that it meanifully > generalizes to any other capability. > > I can in the sandbox today create a user namespace and then set > max_net_namespaces to 0, and drop CAP_NET_RAW and that blocks > the attack. (Possibly with a little spice to prevent a suid root > program from reacquiring CAP_NET_RAW). > This is problematic since you are expecting the user-namespace creator to perform this operation and then block the child process from creating the user-namespace. This is similar to making user-namespace creation a privileged operation discussed previously. > So while your solution doesn't look horrible especially if it can be > done at a user namespace level so the restrictions can be limited to a > single sandbox. I am not at all certain that the capabilities is the > proper place to limit code reachability. > > I would very much like to see which capabilities that are available with > ns_capable, are more meaningful to limit than just dropping the > capability during sandbox creation and denying the creation of the > corresponding namespace. > The primary assumption in this approach is that we can drop capabilities before running the workload and then not allowing workload to create the user-namespace. This does not work for cases where workload needs to create user-namespaces. > CAP_NET_RAW is one. Are there any other capabilities that are > meanginful to limit? > There are currently 37 capabilities and I see many of those are currently namespace aware (with ns_capable() call). Also there seems to be disproportionate amount of capable() to ns_capable() calls. This could be a result of not every feature available kernel-wide being namespace aware/capable etc. and this will evolve and mature i.e. ns_capable() will continue to grow where this would be applicable. Also Probably I'm the wrong person to ask this question to since I understand networking more than anything else. However, the main point is that we cannot predict which vulnerability is going to get published tomorrow networking or non-networking, so having a tool that gives controls to admin while allowing user-namespace creation is super useful. thanks, --mahesh.. > Eric
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