Date: Thu, 09 Nov 2017 15:58:43 -0600 From: ebiederm@...ssion.com (Eric W. Biederman) To: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.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 "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). 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. CAP_NET_RAW is one. Are there any other capabilities that are meanginful to limit? Eric
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