Re: [PATCHv3 0/2] capability controlled user-namespaces

[Michael Kerrisk (man-pages)]

Hello Mahesh,

On 27 December 2017 at 18:09, Mahesh Bandewar (àààà ààààààà)
<maheshb@xxxxxxxxxx> wrote:
> Hello James,
>
> Seems like I missed your name to be added into the review of this
> patch series. Would you be willing be pull this into the security
> tree? Serge Hallyn has already ACKed it.

We seem to have no formal documentation/specification of this feature.
I think that should be written up before this patch goes into
mainline...

Cheers,

Michael


>
> On Tue, Dec 5, 2017 at 2:30 PM, Mahesh Bandewar <mahesh@xxxxxxxxxxxx> wrote:
>> From: Mahesh Bandewar <maheshb@xxxxxxxxxx>
>>
>> TL;DR version
>> -------------
>> Creating a sandbox environment with namespaces is challenging
>> considering what these sandboxed processes can engage into. e.g.
>> CVE-2017-6074, CVE-2017-7184, CVE-2017-7308 etc. just to name few.
>> Current form of user-namespaces, however, if changed a bit can allow
>> us to create a sandbox environment without locking down user-
>> namespaces.
>>
>> Detailed version
>> ----------------
>>
>> Problem
>> -------
>> User-namespaces in the current form have increased the attack surface as
>> any process can acquire capabilities which are not available to them (by
>> default) by performing combination of clone()/unshare()/setns() syscalls.
>>
>>     #define _GNU_SOURCE
>>     #include <stdio.h>
>>     #include <sched.h>
>>     #include <netinet/in.h>
>>
>>     int main(int ac, char **av)
>>     {
>>         int sock = -1;
>>
>>         printf("Attempting to open RAW socket before unshare()...\n");
>>         sock = socket(AF_INET6, SOCK_RAW, IPPROTO_RAW);
>>         if (sock < 0) {
>>             perror("socket() SOCK_RAW failed: ");
>>         } else {
>>             printf("Successfully opened RAW-Sock before unshare().\n");
>>             close(sock);
>>             sock = -1;
>>         }
>>
>>         if (unshare(CLONE_NEWUSER | CLONE_NEWNET) < 0) {
>>             perror("unshare() failed: ");
>>             return 1;
>>         }
>>
>>         printf("Attempting to open RAW socket after unshare()...\n");
>>         sock = socket(AF_INET6, SOCK_RAW, IPPROTO_RAW);
>>         if (sock < 0) {
>>             perror("socket() SOCK_RAW failed: ");
>>         } else {
>>             printf("Successfully opened RAW-Sock after unshare().\n");
>>             close(sock);
>>             sock = -1;
>>         }
>>
>>         return 0;
>>     }
>>
>> The above example shows how easy it is to acquire NET_RAW capabilities
>> and once acquired, these processes could take benefit of above mentioned
>> or similar issues discovered/undiscovered with malicious intent. Note
>> that this is just an example and the problem/solution is not limited
>> to NET_RAW capability *only*.
>>
>> The easiest fix one can apply here is to lock-down user-namespaces which
>> many of the distros do (i.e. don't allow users to create user namespaces),
>> but unfortunately that prevents everyone from using them.
>>
>> Approach
>> --------
>> Introduce a notion of 'controlled' user-namespaces. Every process on
>> the host is allowed to create user-namespaces (governed by the limit
>> imposed by per-ns sysctl) however, mark user-namespaces created by
>> sandboxed processes as 'controlled'. Use this 'mark' at the time of
>> capability check in conjunction with a global capability whitelist.
>> If the capability is not whitelisted, processes that belong to
>> controlled user-namespaces will not be allowed.
>>
>> Once a user-ns is marked as 'controlled'; all its child user-
>> namespaces are marked as 'controlled' too.
>>
>> A global whitelist is list of capabilities governed by the
>> sysctl which is available to (privileged) user in init-ns to modify
>> while it's applicable to all controlled user-namespaces on the host.
>>
>> Marking user-namespaces controlled without modifying the whitelist is
>> equivalent of the current behavior. The default value of whitelist includes
>> all capabilities so that the compatibility is maintained. However it gives
>> admins fine-grained ability to control various capabilities system wide
>> without locking down user-namespaces.
>>
>> Please see individual patches in this series.
>>
>> Mahesh Bandewar (2):
>>   capability: introduce sysctl for controlled user-ns capability whitelist
>>   userns: control capabilities of some user namespaces
>>
>>  Documentation/sysctl/kernel.txt | 21 +++++++++++++++++
>>  include/linux/capability.h      |  7 ++++++
>>  include/linux/user_namespace.h  | 25 ++++++++++++++++++++
>>  kernel/capability.c             | 52 +++++++++++++++++++++++++++++++++++++++++
>>  kernel/sysctl.c                 |  5 ++++
>>  kernel/user_namespace.c         |  4 ++++
>>  security/commoncap.c            |  8 +++++++
>>  7 files changed, 122 insertions(+)
>>
>> --
>> 2.15.0.531.g2ccb3012c9-goog
>>
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-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/