3 files changed, 125 insertions, 8 deletions
diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst
new file mode 100644
index 000000000000..212434ef65ad
--- /dev/null
+++ b/Documentation/admin-guide/LSM/SafeSetID.rst
@@ -0,0 +1,107 @@
+=========
+SafeSetID
+=========
+SafeSetID is an LSM module that gates the setid family of syscalls to restrict
+UID/GID transitions from a given UID/GID to only those approved by a
+system-wide whitelist. These restrictions also prohibit the given UIDs/GIDs
+from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
+allowing a user to set up user namespace UID mappings.
+
+
+Background
+==========
+In absence of file capabilities, processes spawned on a Linux system that need
+to switch to a different user must be spawned with CAP_SETUID privileges.
+CAP_SETUID is granted to programs running as root or those running as a non-root
+user that have been explicitly given the CAP_SETUID runtime capability. It is
+often preferable to use Linux runtime capabilities rather than file
+capabilities, since using file capabilities to run a program with elevated
+privileges opens up possible security holes since any user with access to the
+file can exec() that program to gain the elevated privileges.
+
+While it is possible to implement a tree of processes by giving full
+CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a
+tree of processes under non-root user(s) in the first place. Specifically,
+since CAP_SETUID allows changing to any user on the system, including the root
+user, it is an overpowered capability for what is needed in this scenario,
+especially since programs often only call setuid() to drop privileges to a
+lesser-privileged user -- not elevate privileges. Unfortunately, there is no
+generally feasible way in Linux to restrict the potential UIDs that a user can
+switch to through setuid() beyond allowing a switch to any user on the system.
+This SafeSetID LSM seeks to provide a solution for restricting setid
+capabilities in such a way.
+
+The main use case for this LSM is to allow a non-root program to transition to
+other untrusted uids without full blown CAP_SETUID capabilities. The non-root
+program would still need CAP_SETUID to do any kind of transition, but the
+additional restrictions imposed by this LSM would mean it is a "safer" version
+of CAP_SETUID since the non-root program cannot take advantage of CAP_SETUID to
+do any unapproved actions (e.g. setuid to uid 0 or create/enter new user
+namespace). The higher level goal is to allow for uid-based sandboxing of system
+services without having to give out CAP_SETUID all over the place just so that
+non-root programs can drop to even-lesser-privileged uids. This is especially
+relevant when one non-root daemon on the system should be allowed to spawn other
+processes as different uids, but its undesirable to give the daemon a
+basically-root-equivalent CAP_SETUID.
+
+
+Other Approaches Considered
+===========================
+
+Solve this problem in userspace
+-------------------------------
+For candidate applications that would like to have restricted setid capabilities
+as implemented in this LSM, an alternative option would be to simply take away
+setid capabilities from the application completely and refactor the process
+spawning semantics in the application (e.g. by using a privileged helper program
+to do process spawning and UID/GID transitions). Unfortunately, there are a
+number of semantics around process spawning that would be affected by this, such
+as fork() calls where the program doesn???t immediately call exec() after the
+fork(), parent processes specifying custom environment variables or command line
+args for spawned child processes, or inheritance of file handles across a
+fork()/exec(). Because of this, as solution that uses a privileged helper in
+userspace would likely be less appealing to incorporate into existing projects
+that rely on certain process-spawning semantics in Linux.
+
+Use user namespaces
+-------------------
+Another possible approach would be to run a given process tree in its own user
+namespace and give programs in the tree setid capabilities. In this way,
+programs in the tree could change to any desired UID/GID in the context of their
+own user namespace, and only approved UIDs/GIDs could be mapped back to the
+initial system user namespace, affectively preventing privilege escalation.
+Unfortunately, it is not generally feasible to use user namespaces in isolation,
+without pairing them with other namespace types, which is not always an option.
+Linux checks for capabilities based off of the user namespace that ???owns??? some
+entity. For example, Linux has the notion that network namespaces are owned by
+the user namespace in which they were created. A consequence of this is that
+capability checks for access to a given network namespace are done by checking
+whether a task has the given capability in the context of the user namespace
+that owns the network namespace -- not necessarily the user namespace under
+which the given task runs. Therefore spawning a process in a new user namespace
+effectively prevents it from accessing the network namespace owned by the
+initial namespace. This is a deal-breaker for any application that expects to
+retain the CAP_NET_ADMIN capability for the purpose of adjusting network
+configurations. Using user namespaces in isolation causes problems regarding
+other system interactions, including use of pid namespaces and device creation.
+
+Use an existing LSM
+-------------------
+None of the other in-tree LSMs have the capability to gate setid transitions, or
+even employ the security_task_fix_setuid hook at all. SELinux says of that hook:
+"Since setuid only affects the current process, and since the SELinux controls
+are not based on the Linux identity attributes, SELinux does not need to control
+this operation."
+
+
+Directions for use
+==================
+This LSM hooks the setid syscalls to make sure transitions are allowed if an
+applicable restriction policy is in place. Policies are configured through
+securityfs by writing to the safesetid/add_whitelist_policy and
+safesetid/flush_whitelist_policies files at the location where securityfs is
+mounted. The format for adding a policy is '<UID>:<UID>', using literal
+numbers, such as '123:456'. To flush the policies, any write to the file is
+sufficient. Again, configuring a policy for a UID will prevent that UID from
+obtaining auxiliary setid privileges, such as allowing a user to set up user
+namespace UID mappings.
diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst
index c980dfe9abf1..a6ba95fbaa9f 100644
--- a/Documentation/admin-guide/LSM/index.rst
+++ b/Documentation/admin-guide/LSM/index.rst
@@ -17,9 +17,8 @@ MAC extensions, other extensions can be built using the LSM to provide
 specific changes to system operation when these tweaks are not available
 in the core functionality of Linux itself.
 
-Without a specific LSM built into the kernel, the default LSM will be the
-Linux capabilities system. Most LSMs choose to extend the capabilities
-system, building their checks on top of the defined capability hooks.
+The Linux capabilities modules will always be included. This may be
+followed by any number of "minor" modules and at most one "major" module.
 For more details on capabilities, see ``capabilities(7)`` in the Linux
 man-pages project.
 
@@ -30,6 +29,14 @@ order in which checks are made. The capability module will always
 be first, followed by any "minor" modules (e.g. Yama) and then
 the one "major" module (e.g. SELinux) if there is one configured.
 
+Process attributes associated with "major" security modules should
+be accessed and maintained using the special files in ``/proc/.../attr``.
+A security module may maintain a module specific subdirectory there,
+named after the module. ``/proc/.../attr/smack`` is provided by the Smack
+security module and contains all its special files. The files directly
+in ``/proc/.../attr`` remain as legacy interfaces for modules that provide
+subdirectories.
+
 .. toctree::
    :maxdepth: 1
 
@@ -39,3 +46,4 @@ the one "major" module (e.g. SELinux) if there is one configured.
    Smack
    tomoyo
    Yama
+   SafeSetID
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a422560fbc15..42379633801f 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2333,6 +2333,10 @@
 
 	lsm.debug	[SECURITY] Enable LSM initialization debugging output.
 
+	lsm=lsm1,...,lsmN
+			[SECURITY] Choose order of LSM initialization. This
+			overrides CONFIG_LSM, and the "security=" parameter.
+
 	machvec=	[IA-64] Force the use of a particular machine-vector
 			(machvec) in a generic kernel.
 			Example: machvec=hpzx1_swiotlb
@@ -4110,11 +4114,9 @@
 			Note: increases power consumption, thus should only be
 			enabled if running jitter sensitive (HPC/RT) workloads.
 
-	security=	[SECURITY] Choose a security module to enable at boot.
-			If this boot parameter is not specified, only the first
-			security module asking for security registration will be
-			loaded. An invalid security module name will be treated
-			as if no module has been chosen.
+	security=	[SECURITY] Choose a legacy "major" security module to
+			enable at boot. This has been deprecated by the
+			"lsm=" parameter.
 
 	selinux=	[SELINUX] Disable or enable SELinux at boot time.
 			Format: { "0" | "1" }