• [kernel-hardening] [PATCH 0/2] capability controlled user-namespace

    From Serge E. Hallyn@21:1/5 to All on Mon Oct 2 22:50:10 2017
    Quoting Mahesh Bandewar (mahesh@bandewar.net):
    From: Mahesh Bandewar <maheshb@google.com>

    [Same as the previous RFC series sent on 9/21]

    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
    ----------------

    Hi,

    still struggling with how I feel about the idea in general.

    So is the intent mainly that if/when there comes an 0-day which allows
    users with CAP_NET_ADMIN in any namespace to gain privilege on the host,
    then this can be used as a stop-gap measure until there is a proper fix?

    Otherwise, do you have any guidance for how people should use this?

    IMO it should be heavily discouraged to use this tool as a regular
    day to day configuration, as I'm not sure there is any "educated"
    decision to be made, even by those who are in the know, about what
    to put in this set.

    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 | 4 ++++
    include/linux/user_namespace.h | 20 ++++++++++++++++
    kernel/capability.c | 52 +++++++++++++++++++++++++++++++++++++++++
    kernel/sysctl.c | 5 ++++
    kernel/user_namespace.c | 3 +++
    security/commoncap.c | 8 +++++++
    7 files changed, 113 insertions(+)

    --
    2.14.2.822.g60be5d43e6-goog

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  • From =?UTF-8?B?TWFoZXNoIEJhbmRld2FyICjgp@21:1/5 to Serge E. Hallyn on Mon Oct 2 22:50:11 2017
    On Mon, Oct 2, 2017 at 10:14 AM, Serge E. Hallyn <serge@hallyn.com> wrote:
    Quoting Mahesh Bandewar (mahesh@bandewar.net):
    From: Mahesh Bandewar <maheshb@google.com>

    [Same as the previous RFC series sent on 9/21]

    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
    ----------------

    Hi,

    still struggling with how I feel about the idea in general.

    So is the intent mainly that if/when there comes an 0-day which allows
    users with CAP_NET_ADMIN in any namespace to gain privilege on the host,
    then this can be used as a stop-gap measure until there is a proper fix?

    Thank for looking at this Serge.

    Yes, but at the same time it's not just limited to NET_ADMIN but could
    be any of the current capabilities.

    Otherwise, do you have any guidance for how people should use this?

    IMO it should be heavily discouraged to use this tool as a regular
    day to day configuration, as I'm not sure there is any "educated"
    decision to be made, even by those who are in the know, about what
    to put in this set.

    I think that really depends on the environment. e.g. in certain
    sandboxes third-part / semi-trusted workload is executed where network
    resource is not used. In that environment I can easily take off
    NET_ADMIN and NET_RAW without affecting anything there. At the same
    time I wont have to worry about 0-day related to these two
    capabilities. I would say the Admins at these places are in the best
    place to decide what they can take-off safely and what they cannot.
    Even if they decide not to take-off anything, having a tool at hand to
    gain control is important when the next 0-day strikes us that can be
    exploited using any of the currently used capabilities.

    However, you are absolutely right in terms of using it as a stop-gap
    measure to protect environment until it's fixed and the capability in
    question can not be safely taken off permanently without hampering
    operations.

    thanks,
    --mahesh..

    [...]

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