1 =========
2 SafeSetID
3 =========
4 SafeSetID is an LSM module that gates the setid family of syscalls to restrict
5 UID/GID transitions from a given UID/GID to only those approved by a
6 system-wide allowlist. These restrictions also prohibit the given UIDs/GIDs
7 from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
8 allowing a user to set up user namespace UID/GID mappings.
9
10
11 Background
12 ==========
13 In absence of file capabilities, processes spawned on a Linux system that need
14 to switch to a different user must be spawned with CAP_SETUID privileges.
15 CAP_SETUID is granted to programs running as root or those running as a non-root
16 user that have been explicitly given the CAP_SETUID runtime capability. It is
17 often preferable to use Linux runtime capabilities rather than file
18 capabilities, since using file capabilities to run a program with elevated
19 privileges opens up possible security holes since any user with access to the
20 file can exec() that program to gain the elevated privileges.
21
22 While it is possible to implement a tree of processes by giving full
23 CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a
24 tree of processes under non-root user(s) in the first place. Specifically,
25 since CAP_SETUID allows changing to any user on the system, including the root
26 user, it is an overpowered capability for what is needed in this scenario,
27 especially since programs often only call setuid() to drop privileges to a
28 lesser-privileged user -- not elevate privileges. Unfortunately, there is no
29 generally feasible way in Linux to restrict the potential UIDs that a user can
30 switch to through setuid() beyond allowing a switch to any user on the system.
31 This SafeSetID LSM seeks to provide a solution for restricting setid
32 capabilities in such a way.
33
34 The main use case for this LSM is to allow a non-root program to transition to
35 other untrusted uids without full blown CAP_SETUID capabilities. The non-root
36 program would still need CAP_SETUID to do any kind of transition, but the
37 additional restrictions imposed by this LSM would mean it is a "safer" version
38 of CAP_SETUID since the non-root program cannot take advantage of CAP_SETUID to
39 do any unapproved actions (e.g. setuid to uid 0 or create/enter new user
40 namespace). The higher level goal is to allow for uid-based sandboxing of system
41 services without having to give out CAP_SETUID all over the place just so that
42 non-root programs can drop to even-lesser-privileged uids. This is especially
43 relevant when one non-root daemon on the system should be allowed to spawn other
44 processes as different uids, but its undesirable to give the daemon a
45 basically-root-equivalent CAP_SETUID.
46
47
48 Other Approaches Considered
49 ===========================
50
51 Solve this problem in userspace
52 -------------------------------
53 For candidate applications that would like to have restricted setid capabilities
54 as implemented in this LSM, an alternative option would be to simply take away
55 setid capabilities from the application completely and refactor the process
56 spawning semantics in the application (e.g. by using a privileged helper program
57 to do process spawning and UID/GID transitions). Unfortunately, there are a
58 number of semantics around process spawning that would be affected by this, such
59 as fork() calls where the program doesn't immediately call exec() after the
60 fork(), parent processes specifying custom environment variables or command line
61 args for spawned child processes, or inheritance of file handles across a
62 fork()/exec(). Because of this, as solution that uses a privileged helper in
63 userspace would likely be less appealing to incorporate into existing projects
64 that rely on certain process-spawning semantics in Linux.
65
66 Use user namespaces
67 -------------------
68 Another possible approach would be to run a given process tree in its own user
69 namespace and give programs in the tree setid capabilities. In this way,
70 programs in the tree could change to any desired UID/GID in the context of their
71 own user namespace, and only approved UIDs/GIDs could be mapped back to the
72 initial system user namespace, affectively preventing privilege escalation.
73 Unfortunately, it is not generally feasible to use user namespaces in isolation,
74 without pairing them with other namespace types, which is not always an option.
75 Linux checks for capabilities based off of the user namespace that "owns" some
76 entity. For example, Linux has the notion that network namespaces are owned by
77 the user namespace in which they were created. A consequence of this is that
78 capability checks for access to a given network namespace are done by checking
79 whether a task has the given capability in the context of the user namespace
80 that owns the network namespace -- not necessarily the user namespace under
81 which the given task runs. Therefore spawning a process in a new user namespace
82 effectively prevents it from accessing the network namespace owned by the
83 initial namespace. This is a deal-breaker for any application that expects to
84 retain the CAP_NET_ADMIN capability for the purpose of adjusting network
85 configurations. Using user namespaces in isolation causes problems regarding
86 other system interactions, including use of pid namespaces and device creation.
87
88 Use an existing LSM
89 -------------------
90 None of the other in-tree LSMs have the capability to gate setid transitions, or
91 even employ the security_task_fix_setuid hook at all. SELinux says of that hook:
92 "Since setuid only affects the current process, and since the SELinux controls
93 are not based on the Linux identity attributes, SELinux does not need to control
94 this operation."
95
96
97 Directions for use
98 ==================
99 This LSM hooks the setid syscalls to make sure transitions are allowed if an
100 applicable restriction policy is in place. Policies are configured through
101 securityfs by writing to the safesetid/uid_allowlist_policy and
102 safesetid/gid_allowlist_policy files at the location where securityfs is
103 mounted. The format for adding a policy is '<UID>:<UID>' or '<GID>:<GID>',
104 using literal numbers, and ending with a newline character such as '123:456\n'.
105 Writing an empty string "" will flush the policy. Again, configuring a policy
106 for a UID/GID will prevent that UID/GID from obtaining auxiliary setid
107 privileges, such as allowing a user to set up user namespace UID/GID mappings.
108
109 Note on GID policies and setgroups()
110 ====================================
111 In v5.9 we are adding support for limiting CAP_SETGID privileges as was done
112 previously for CAP_SETUID. However, for compatibility with common sandboxing
113 related code conventions in userspace, we currently allow arbitrary
114 setgroups() calls for processes with CAP_SETGID restrictions. Until we add
115 support in a future release for restricting setgroups() calls, these GID
116 policies add no meaningful security. setgroups() restrictions will be enforced
117 once we have the policy checking code in place, which will rely on GID policy
118 configuration code added in v5.9.
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