Adds Documentation/smack-namespace.txt. Signed-off-by: Lukasz Pawelczyk <l.pawelczyk@xxxxxxxxxxx> Reviewed-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx> --- 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..120314d --- /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 a226416..6effb1a 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -9377,6 +9377,7 @@ 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. -- 2.4.3 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
