1.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
  2
  3===========================
  4BPF_PROG_TYPE_CGROUP_SYSCTL
  5===========================
  6
  7This document describes ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program type that
  8provides cgroup-bpf hook for sysctl.
  9
 10The hook has to be attached to a cgroup and will be called every time a
 11process inside that cgroup tries to read from or write to sysctl knob in proc.
 12
 131. 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
 192. Context
 20**********
 21
 22``BPF_PROG_TYPE_CGROUP_SYSCTL`` provides access to the following context from
 23BPF 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
 44See `linux/bpf.h`_ for more details on how context field can be accessed.
 45
 463. Return code
 47**************
 48
 49``BPF_PROG_TYPE_CGROUP_SYSCTL`` program must return one of the following
 50return codes:
 51
 52* ``0`` means "reject access to sysctl";
 53* ``1`` means "proceed with access".
 54
 55If program returns ``0`` user space will get ``-1`` from ``read(2)`` or
 56``write(2)`` and ``errno`` will be set to ``EPERM``.
 57
 584. Helpers
 59**********
 60
 61Since sysctl knob is represented by a name and a value, sysctl specific BPF
 62helpers 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
 83BPF program sees sysctl value same way as user space does in proc filesystem,
 84i.e. as a string. Since many sysctl values represent an integer or a vector
 85of integers, the following helpers can be used to get numeric value from the
 86string:
 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
 93See `linux/bpf.h`_ for more details on helpers described here.
 94
 955. Examples
 96***********
 97
 98See `test_sysctl_prog.c`_ for an example of BPF program in C that access
 99sysctl name and value, parses string value to get vector of integers and uses
100the result to make decision whether to allow or deny access to sysctl.
101
1026. Notes
103********
104
105``BPF_PROG_TYPE_CGROUP_SYSCTL`` is intended to be used in **trusted** root
106environment, for example to monitor sysctl usage or catch unreasonable values
107an application, running as root in a separate cgroup, is trying to set.
108
109Since `task_dfl_cgroup(current)` is called at `sys_read` / `sys_write` time it
110may return results different from that at `sys_open` time, i.e. process that
111opened sysctl file in proc filesystem may differ from process that is trying
112to read from / write to it and two such processes may run in different
113cgroups, what means ``BPF_PROG_TYPE_CGROUP_SYSCTL`` should not be used as a
114security mechanism to limit sysctl usage.
115
116As with any cgroup-bpf program additional care should be taken if an
117application running as root in a cgroup should not be allowed to
118detach/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