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 =========
 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 allowlist. 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/GID 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/uid_allowlist_policy and
 safesetid/gid_allowlist_policy files at the location where securityfs is
 mounted. The format for adding a policy is '<UID>:<UID>' or '<GID>:<GID>',
 using literal numbers, and ending with a newline character such as '123:456\n'.
 Writing an empty string "" will flush the policy. Again, configuring a policy
 for a UID/GID will prevent that UID/GID from obtaining auxiliary setid
 privileges, such as allowing a user to set up user namespace UID/GID mappings.
 
 Note on GID policies and setgroups()
 ====================================
 In v5.9 we are adding support for limiting CAP_SETGID privileges as was done
 previously for CAP_SETUID. However, for compatibility with common sandboxing
 related code conventions in userspace, we currently allow arbitrary
 setgroups() calls for processes with CAP_SETGID restrictions. Until we add
 support in a future release for restricting setgroups() calls, these GID
 policies add no meaningful security. setgroups() restrictions will be enforced
 once we have the policy checking code in place, which will rely on GID policy
 configuration code added in v5.9.

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