Re: [PATCH v4 11/11] smack: documentation for the Smack namespace

[Casey Schaufler <casey-iSGtlc1asvQWG2LlvL+J4A@public.gmane.org>]

On 10/14/2015 5:42 AM, Lukasz Pawelczyk wrote:
> Adds Documentation/smack-namespace.txt.
>
> Signed-off-by: Lukasz Pawelczyk <l.pawelczyk-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>
> Reviewed-by: Casey Schaufler <casey-iSGtlc1asvQWG2LlvL+J4A@public.gmane.org>

Acked-by: Casey Schaufler <casey-iSGtlc1asvQWG2LlvL+J4A@public.gmane.org>


> ---
>  Documentation/security/00-INDEX            |   2 +
>  Documentation/security/Smack-namespace.txt | 231 +++++++++++++++++++++++++++++
>  MAINTAINERS                                |   1 +
>  security/smack/Kconfig                     |   2 +
>  4 files changed, 236 insertions(+)
>  create mode 100644 Documentation/security/Smack-namespace.txt
>
> diff --git a/Documentation/security/00-INDEX b/Documentation/security/00-INDEX
> index 45c82fd..c03a220 100644
> --- a/Documentation/security/00-INDEX
> +++ b/Documentation/security/00-INDEX
> @@ -6,6 +6,8 @@ SELinux.txt
>  	- how to get started with the SELinux security enhancement.
>  Smack.txt
>  	- documentation on the Smack Linux Security Module.
> +Smack-namespace.txt
> +	- documentation on the Smack namespace implementation.
>  Yama.txt
>  	- documentation on the Yama Linux Security Module.
>  apparmor.txt
> diff --git a/Documentation/security/Smack-namespace.txt b/Documentation/security/Smack-namespace.txt
> new file mode 100644
> index 0000000..5304355
> --- /dev/null
> +++ b/Documentation/security/Smack-namespace.txt
> @@ -0,0 +1,231 @@
> +
> +                                      "Quis custodiet ipsos custodes?"
> +                                                 - Satires of Juvenal
> +
> +
> +--- What is a Smack namespace ---
> +
> +Smack namespace was developed to make it possible for Smack to work
> +nicely with Linux containers where there is a full operating system
> +with its own init inside the namespace. Such a system working with
> +Smack expects to have at least partially working SMACK_MAC_ADMIN to be
> +able to change labels of processes and files. This is required to be
> +able to securely start applications under the control of Smack and
> +manage their access rights.
> +
> +It was implemented using new LSM hooks added to the user namespace
> +that were developed together with Smack namespace.
> +
> +
> +--- Design ideas ---
> +
> +"Smack namespace" is rather "Smack labels namespace" as not the whole
> +MAC is namespaced, only the labels. There is a great analogy between
> +Smack labels namespace and the user namespace part that remaps UIDs.
> +
> +The idea is to create a map of labels for a namespace so the namespace
> +is only allowed to use those labels. Smack rules are always the same
> +as in the init namespace (limited only by what labels are mapped) and
> +cannot be manipulated from the child namespace. The map is actually
> +only for labels' names. The underlying structures for labels remain
> +the same. The filesystem also stores the "unmapped" labels from the
> +init namespace.
> +
> +Let's say we have those labels in the init namespace:
> +label1
> +label2
> +label3
> +
> +and those rules:
> +label1 label2 rwx
> +label1 label3 rwx
> +label2 label3 rwx
> +
> +We create a map for a namespace:
> +label1 -> mapped1
> +label2 -> mapped2
> +
> +This means that 'label3' is completely invisible in the namespace. As if
> +it didn't exist. All the rules that include it are ignored.
> +
> +Effectively in the namespace we have only one rule:
> +mapped1 mapped2 rwx
> +
> +Which in reality is:
> +label1 label2 rwx
> +
> +All requests to access an object with a 'label3' will be denied. If it
> +ever comes to a situation where 'label3' would have to be printed
> +(e.g. reading an exec or mmap label from a file to which we have
> +access) then huh sign '?' will be printed instead.
> +
> +All the operations in the namespace on the remaining labels will have
> +to be performed using their mapped names. Things like changing own
> +process's label, changing filesystem label. Labels will also be
> +printed with their mapped names.
> +
> +You cannot import new labels in a namespace. Every operation that
> +would do so in an init namespace will return an error in the child
> +namespace. You cannot assign an unmapped or not existing label to an
> +object. You can only operate on labels that have been explicitly
> +mapped.
> +
> +
> +--- Capabilities ---
> +
> +Enabling Smack related capabilities (CAP_MAC_ADMIN and
> +CAP_MAC_OVERRIDE) is main goal of Smack namespace, so it can work
> +properly in the container. And those capabilities do work to some
> +extent. In several places where capabilities are checked compatibility
> +with Smack namespace has been introduced. Capabilities are of course
> +limited to operate only on mapped labels.
> +
> +CAP_MAC_OVERRIDE works fully, will allow you to ignore Smack access
> +rules, but only between objects that have labels mapped. So in the
> +example above having this CAP will allow e.g. label2 to write to
> +label1, but will not allow any access to label3.
> +
> +With CAP_MAC_ADMIN the following operations has been allowed inside
> +the namespace:
> +- setting and removing xattr on files, including the security.* ones
> +- setting process's own label (/proc/self/attr/current)
> +- mounting in a privileged Smack mode, which means one can specify
> +  additional mount options like: smackfsdef, smackfsfloor etc.
> +
> +Again this is also allowed only on the mapped labels. Labels on the
> +filesystem will be stored in unmapped form so they are preserved
> +through reboots.
> +
> +Such a namespace construct allows e.g. systemd (with Smack support)
> +working in a container to assign labels properly to daemons and other
> +processes.
> +
> +
> +--- Usage ---
> +
> +Smack namespace is written using LSM hooks inside user namespace. That
> +means it's connected to it.
> +
> +To create a new Smack namespace you need to unshare() user namespace
> +as usual. If that is all you do though, than there is no difference to
> +what is now. To activate the Smack namespace you need to fill the
> +labels' map. It is in a file /proc/$PID/attr/label_map.
> +
> +By default the map is empty and Smack namespaces are inactive (labels
> +are taken directly from a parent namespace). It also means that the
> +Smack capabilities will be inactive. After you fill the map it starts
> +to take effect in the namespace and Smack capabilities (only on mapped
> +labels) start to work.
> +
> +Due to the way Smack works only CAP_MAC_ADMIN from the parent
> +namespace (init_user_ns for now, see the "Current limitations" below)
> +is allowed to fill the map. That means that an unprivileged user is
> +still allowed to create the user namespace but it will not be able to
> +fill the labels' map (activate Smack namespace). An administrator
> +intervention is required.
> +
> +The attr_map write format is:
> +unmapped_label mapped_label
> +
> +When reading the file it shows an active map for a namespace the
> +process in question is in in the format:
> +unmapped_label -> mapped_label
> +
> +If the label_map file is empty it means the namespace is not mapped
> +and Smack namespace is inactive (no mappings, MAC related capabilities
> +behave as they did before, meaning they are active only in
> +init_user_ns). For init_user_ns the map will always be empty.
> +
> +Writing to the map file is not disabled after the first write as it is
> +in uid_map. For Smack we have no means to map ranges of labels, hence
> +it can really be advantageous to be able to expand the map later
> +on. But you can only add to the map. You cannot remove already mapped
> +labels. You cannot change the already existing mappings. Also mappings
> +has to be 1-1. All requests to create a map where either the unmapped
> +or the mapped label already exists in the map will be denied.
> +
> +setns() with Smack namespace active has an additional check that the
> +label of a process that is calling setns() has to be already mapped in
> +the target Smack namespace for the call to succeed.
> +
> +
> +--- Special labels ---
> +
> +Smack is using some special labels that have built-in rules. Things
> +like floor '_', dash '^', star '*', etc. Those labels are not
> +automatically mapped to the namespace. Moreover, you can choose to map
> +a different label from the init namespace to behave e.g. like floor
> +inside the namespace.
> +
> +Let's say we have no rules and those labels in the init namespace:
> +_
> +floor_to_be
> +label
> +
> +Both 'label' and 'floor_to_be' can read objects with '_'. But they
> +have no access rights to each other.
> +
> +Now let's create a map like this:
> +_             ordinary_label
> +floor_to_be   _
> +label         mapped
> +
> +Right now label 'mapped' can read label '_' which means that
> +effectively inside this namespace label 'label' has gained read access
> +to the 'floor_to_be'. The label 'ordinary_label' is exactly it, an
> +ordinary label that the built-in rules no longer apply to inside the
> +namespace.
> +
> +To sum up, special labels in the namespace behave the same as in the
> +init namespace. Not the original special labels though, but the ones
> +we map to specials. This is the only case where a namespace can have
> +access rights the init namespace does not have (like the 'label' to
> +'floor_to_be' in the example above).
> +
> +Of course mappings like these are perfectly legal:
> +_   _
> +*   *
> +^   ^
> +
> +
> +--- Current limitations ---
> +
> +The Smack namespace is not hierarchical yet. It is currently not
> +possible to fill a label_map of a nested user namespace (you can still
> +create nested user namespace, it will just inherit its parent's map
> +and won't have active Smack capabilities). When hierarchy will be
> +implemented the process creating another namespace will be allowed to
> +map only labels that it has permission to itself (those that it has in
> +its own map).
> +
> +Special files inside the virtual smackfs needs to be reviewed whether
> +it's beneficial to have some of their functionality namespaced as well
> +(e.g. onlycap, syslog. ambient, etc). This would increase
> +CAP_MAC_ADMIN privileges inside the namespace.
> +
> +
> +--- Error codes ---
> +
> +While working in the namespace patches the error codes has been made
> +to propagate properly from a place they occurred. New error codes has
> +also been introduced for Smack in the context of namespace usage. This
> +is a complete summary of error codes used throughout the Smack now:
> +
> +ENOMEM      and other system errors that might come from low level
> +            kernel functions like memory allocations
> +EOPNOTSUPP  means the underlying system operation is not
> +            supported (eg. getxattr)
> +EINVAL      means invalid syntax (e.g. empty label or one starting
> +            with '-')
> +EEXIST      when creating map means that a label is already mapped
> +EBADR       is used for wrong namespace usage:
> +            - trying to import a label inside a namespace (like trying
> +              to use an unmapped label that would otherwise be imported)
> +            - trying to create a Smack label map in the init namespace
> +ENOENT      when failed to find a label we expected to exist (will not
> +            be propagated to user-space)
> +EPERM       means no permission to operate on an object, e.g. due to
> +            insufficient capabilities or simply because the object
> +            cannot be operated on in the current context
> +EACCESS     when access has been denied due to Smack access checks
> +            (including object being outside of a namespace)
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 797236b..c77be5a 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -9590,6 +9590,7 @@ W:	http://schaufler-ca.com
>  T:	git git://git.gitorious.org/smack-next/kernel.git
>  S:	Maintained
>  F:	Documentation/security/Smack.txt
> +F:	Documentation/security/Smack-namespace.txt
>  F:	security/smack/
>  
>  DRIVERS FOR ADAPTIVE VOLTAGE SCALING (AVS)
> diff --git a/security/smack/Kconfig b/security/smack/Kconfig
> index b19a7fb..a6e0f3f 100644
> --- a/security/smack/Kconfig
> +++ b/security/smack/Kconfig
> @@ -49,4 +49,6 @@ config SECURITY_SMACK_NS
>  	  This enables Smack namespace that makes it possible to map
>  	  specific labels within user namespace (analogously to mapping
>  	  UIDs) and to gain MAC capabilities over them.
> +	  Documentation is availabile here:
> +	  Documentation/security/Smack-namespace.txt
>  	  If you are unsure how to answer this question, answer N.