1 .. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
2
3 ===========================
4 BPF_PROG_TYPE_CGROUP_SYSCTL
5 ===========================
6
7 This document describes ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program type that
8 provides cgroup-bpf hook for sysctl.
9
10 The hook has to be attached to a cgroup and will be called every time a
11 process inside that cgroup tries to read from or write to sysctl knob in proc.
12
13 1. Attach type
14 **************
15
16 ``BPF_CGROUP_SYSCTL`` attach type has to be used to attach
17 ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program to a cgroup.
18
19 2. Context
20 **********
21
22 ``BPF_PROG_TYPE_CGROUP_SYSCTL`` provides access to the following context from
23 BPF program::
24
25 struct bpf_sysctl {
26 __u32 write;
27 __u32 file_pos;
28 };
29
30 * ``write`` indicates whether sysctl value is being read (``0``) or written
31 (``1``). This field is read-only.
32
33 * ``file_pos`` indicates file position sysctl is being accessed at, read
34 or written. This field is read-write. Writing to the field sets the starting
35 position in sysctl proc file ``read(2)`` will be reading from or ``write(2)``
36 will be writing to. Writing zero to the field can be used e.g. to override
37 whole sysctl value by ``bpf_sysctl_set_new_value()`` on ``write(2)`` even
38 when it's called by user space on ``file_pos > 0``. Writing non-zero
39 value to the field can be used to access part of sysctl value starting from
40 specified ``file_pos``. Not all sysctl support access with ``file_pos !=
41 0``, e.g. writes to numeric sysctl entries must always be at file position
42 ``0``. See also ``kernel.sysctl_writes_strict`` sysctl.
43
44 See `linux/bpf.h`_ for more details on how context field can be accessed.
45
46 3. Return code
47 **************
48
49 ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program must return one of the following
50 return codes:
51
52 * ``0`` means "reject access to sysctl";
53 * ``1`` means "proceed with access".
54
55 If program returns ``0`` user space will get ``-1`` from ``read(2)`` or
56 ``write(2)`` and ``errno`` will be set to ``EPERM``.
57
58 4. Helpers
59 **********
60
61 Since sysctl knob is represented by a name and a value, sysctl specific BPF
62 helpers focus on providing access to these properties:
63
64 * ``bpf_sysctl_get_name()`` to get sysctl name as it is visible in
65 ``/proc/sys`` into provided by BPF program buffer;
66
67 * ``bpf_sysctl_get_current_value()`` to get string value currently held by
68 sysctl into provided by BPF program buffer. This helper is available on both
69 ``read(2)`` from and ``write(2)`` to sysctl;
70
71 * ``bpf_sysctl_get_new_value()`` to get new string value currently being
72 written to sysctl before actual write happens. This helper can be used only
73 on ``ctx->write == 1``;
74
75 * ``bpf_sysctl_set_new_value()`` to override new string value currently being
76 written to sysctl before actual write happens. Sysctl value will be
77 overridden starting from the current ``ctx->file_pos``. If the whole value
78 has to be overridden BPF program can set ``file_pos`` to zero before calling
79 to the helper. This helper can be used only on ``ctx->write == 1``. New
80 string value set by the helper is treated and verified by kernel same way as
81 an equivalent string passed by user space.
82
83 BPF program sees sysctl value same way as user space does in proc filesystem,
84 i.e. as a string. Since many sysctl values represent an integer or a vector
85 of integers, the following helpers can be used to get numeric value from the
86 string:
87
88 * ``bpf_strtol()`` to convert initial part of the string to long integer
89 similar to user space `strtol(3)`_;
90 * ``bpf_strtoul()`` to convert initial part of the string to unsigned long
91 integer similar to user space `strtoul(3)`_;
92
93 See `linux/bpf.h`_ for more details on helpers described here.
94
95 5. Examples
96 ***********
97
98 See `test_sysctl_prog.c`_ for an example of BPF program in C that access
99 sysctl name and value, parses string value to get vector of integers and uses
100 the result to make decision whether to allow or deny access to sysctl.
101
102 6. Notes
103 ********
104
105 ``BPF_PROG_TYPE_CGROUP_SYSCTL`` is intended to be used in **trusted** root
106 environment, for example to monitor sysctl usage or catch unreasonable values
107 an application, running as root in a separate cgroup, is trying to set.
108
109 Since `task_dfl_cgroup(current)` is called at `sys_read` / `sys_write` time it
110 may return results different from that at `sys_open` time, i.e. process that
111 opened sysctl file in proc filesystem may differ from process that is trying
112 to read from / write to it and two such processes may run in different
113 cgroups, what means ``BPF_PROG_TYPE_CGROUP_SYSCTL`` should not be used as a
114 security mechanism to limit sysctl usage.
115
116 As with any cgroup-bpf program additional care should be taken if an
117 application running as root in a cgroup should not be allowed to
118 detach/replace BPF program attached by administrator.
119
120 .. Links
121 .. _linux/bpf.h: ../../include/uapi/linux/bpf.h
122 .. _strtol(3): http://man7.org/linux/man-pages/man3/strtol.3p.html
123 .. _strtoul(3): http://man7.org/linux/man-pages/man3/strtoul.3p.html
124 .. _test_sysctl_prog.c:
125 ../../tools/testing/selftests/bpf/progs/test_sysctl_prog.c
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