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Date: Wed, 27 Dec 2017 21:23:23 +0100
From: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com>
Cc: James Morris <james.l.morris@...cle.com>, 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>, Serge Hallyn <serge@...lyn.com>,
"Eric W . Biederman" <ebiederm@...ssion.com>, Eric Dumazet <edumazet@...gle.com>,
David Miller <davem@...emloft.net>, Mahesh Bandewar <mahesh@...dewar.net>
Subject: Re: [PATCHv3 0/2] capability controlled user-namespaces
Hello Mahesh,
On 27 December 2017 at 18:09, Mahesh Bandewar (महेश बंडेवार)
<maheshb@...gle.com> 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@...dewar.net> wrote:
>> From: Mahesh Bandewar <maheshb@...gle.com>
>>
>> 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/
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