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Date: Tue, 2 Jan 2018 17:30:34 -0800
From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com>
To: James Morris <james.l.morris@...cle.com>
Cc: 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
On Sat, Dec 30, 2017 at 12:31 AM, James Morris
<james.l.morris@...cle.com> wrote:
> On Wed, 27 Dec 2017, Mahesh Bandewar (महेश बंडेवार) 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.
>
> Sure!
>
Thank you James.
>
>>
>> Thanks,
>> --mahesh..
>>
>> 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
>> >
>>
>
> --
> James Morris
> <james.l.morris@...cle.com>
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