1pam_cgfs(8) pam_cgfs(8)
2
6 pam_cgfs - cgroup management for unprivileged LXC containers.
7
9 pam_cgfs.so {-c kernel_controller,name=named_controller}
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12 LXC has supported fully unprivileged containers since LXC 1.0. Fully
13 unprivileged containers are the safest containers and are run by normal
14 (non-root) users. This is achieved by using user namespaces by mapping
15 between a range of UIDs and GIDs on the host to a different (unprivi‐
16 leged) range of UIDs and GIDs in the container. That means the uid 0
17 (root) in the container is mapped to an unprivileged user id (something
18 like 1000000) outside of the container and only has rights on resources
19 that it owns itself.
20 Cgroup management of fully unprivileged containers means restricting
22 the resources used by these containers like limiting the CPU usage of a
23 container, or the number of processes it is allowed to spawn, or the
24 memory it is allowed to consume. It is clear that the fully unprivi‐
25 leged containers are run by normal users and there is a need to limit
26 and manage resource consumption among the containers. But unprivileged
27 cgroup management is not easy with most init systems. So, the
28 pam_cgfs.so came into existence.
29 The pam_cgfs.so module can handle pure cgroupfs v1
31 (/sys/fs/cgroup/$controller) and mixed mounts, where some controllers
32 are mounted in a standard cgroupfs v1 hierarchy (/sys/fs/cgroup/$con‐
33 troller) and others in cgroupfs v2 hierarchy (/sys/fs/cgroup/unified).
34 Writeable cgroups are either created for all controllers or, if speci‐
35 fied, for only controllers listed as arguments on the command line.
36 Pure cgroup v2 mount is not covered by the pam_cgfs.so module.
37 The cgroup created user/$user/n will be for the nth session under
39 cgroup kernel controller hierarchy.
40 Systems with a systemd init system are treated specifically, both with
42 respect to cgroupfs v1 and cgroupfs v2. For both, cgroupfs v1 and
43 cgroupfs v2, the module checks whether systemd already placed the user
44 in a cgroup it created user.slice/user-$uid/session-n.scope by checking
45 whether $uid == login uid. If so, the login user chown the ses‐
46 sion-n.scope, else a cgroup is created as outlined above (user/$user/n)
47 and chown it to login uid. If the init system has already placed the
48 login user inside a session specific group, the pam_cgfs.so module is
49 smart enough to detect it and re-use the cgroup.
50 In essence, the pam_cgfs.so module takes care of placing unprivileged
52 (non-root) users into writable cgroups at login and also cleaning up
53 these cgroup hierarchies on logout, so they are free to delegate re‐
54 sources to containers as needed that have been provided to them.
55
57 -c controller-list
58 Takes a string argument which sets the list of kernel con‐
59 trollers and named controllers delimited by commas in-between
60 “,”. Named controllers need to be specified in the form
61 “name=$namedcontroller”. Can use “all” enable all cgroup re‐
62 source controller hierarchies. Specifying “all” and other con‐
63 trollers explicitly returns PAM_SESSION_ERR.
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66 Only session module type is provided (and needed).
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69 PAM_SUCCESS
70 Writeable cgroups have been created for the user.
71 PAM_SESSION_ERR
73 Failed to create writable cgroups for the user.
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76 /etc/pam.d/common-session{,-noninteractive}
77 Default configuration is added at the end of these files.
78
80 session optional pam_cgfs.so -c freezer,memory,named=systemd
81 # default configuration
82 # user writable cgroups are created under freezer, memory and named cgroup systemd hierarchies.
83 # /sys/fs/cgroup/$controller/user/$user/n for freezer,memory.
84 # /sys/fs/cgroup/systemd/user.slice/user-$uid/session-n.scope for systemd.
85 session optional pam_cgfs.so -c all
87 # user writable cgroups are created under all cgroup controllers.
88 session optional pam_cgfs.so -c all,memory,freezer
90 # invalid argument and returns PAM_SESSION_ERR
91
94 lxc-cgroup(1), cgroups(7), user_namespaces(7), namespaces(7), pam(8)
95
97 Venkata Harshavardhan Reddy Allu <venkataharshavardhan_ven@srmu‐
98 niv.edu.in>
99 2021-09-18 pam_cgfs(8)