1 // SPDX-License-Identifier: GPL-2.0-only
3 * Process number limiting controller for cgroups.
5 * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
6 * after a certain limit is reached.
8 * Since it is trivial to hit the task limit without hitting any kmemcg limits
9 * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
10 * preventable in the scope of a cgroup hierarchy by allowing resource limiting
11 * of the number of tasks in a cgroup.
13 * In order to use the `pids` controller, set the maximum number of tasks in
14 * pids.max (this is not available in the root cgroup for obvious reasons). The
15 * number of processes currently in the cgroup is given by pids.current.
16 * Organisational operations are not blocked by cgroup policies, so it is
17 * possible to have pids.current > pids.max. However, it is not possible to
18 * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
19 * would cause a cgroup policy to be violated.
21 * To set a cgroup to have no limit, set pids.max to "max". This is the default
22 * for all new cgroups (N.B. that PID limits are hierarchical, so the most
23 * stringent limit in the hierarchy is followed).
25 * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
26 * a superset of parent/child/pids.current.
28 * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
31 #include <linux/kernel.h>
32 #include <linux/threads.h>
33 #include <linux/atomic.h>
34 #include <linux/cgroup.h>
35 #include <linux/slab.h>
37 #define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
38 #define PIDS_MAX_STR "max"
41 struct cgroup_subsys_state css;
44 * Use 64-bit types so that we can safely represent "max" as
45 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
50 /* Handle for "pids.events" */
51 struct cgroup_file events_file;
53 /* Number of times fork failed because limit was hit. */
54 atomic64_t events_limit;
57 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
59 return container_of(css, struct pids_cgroup, css);
62 static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
64 return css_pids(pids->css.parent);
67 static struct cgroup_subsys_state *
68 pids_css_alloc(struct cgroup_subsys_state *parent)
70 struct pids_cgroup *pids;
72 pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
74 return ERR_PTR(-ENOMEM);
76 pids->limit = PIDS_MAX;
77 atomic64_set(&pids->counter, 0);
78 atomic64_set(&pids->events_limit, 0);
82 static void pids_css_free(struct cgroup_subsys_state *css)
88 * pids_cancel - uncharge the local pid count
89 * @pids: the pid cgroup state
90 * @num: the number of pids to cancel
92 * This function will WARN if the pid count goes under 0, because such a case is
93 * a bug in the pids controller proper.
95 static void pids_cancel(struct pids_cgroup *pids, int num)
98 * A negative count (or overflow for that matter) is invalid,
99 * and indicates a bug in the `pids` controller proper.
101 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
105 * pids_uncharge - hierarchically uncharge the pid count
106 * @pids: the pid cgroup state
107 * @num: the number of pids to uncharge
109 static void pids_uncharge(struct pids_cgroup *pids, int num)
111 struct pids_cgroup *p;
113 for (p = pids; parent_pids(p); p = parent_pids(p))
118 * pids_charge - hierarchically charge the pid count
119 * @pids: the pid cgroup state
120 * @num: the number of pids to charge
122 * This function does *not* follow the pid limit set. It cannot fail and the new
123 * pid count may exceed the limit. This is only used for reverting failed
124 * attaches, where there is no other way out than violating the limit.
126 static void pids_charge(struct pids_cgroup *pids, int num)
128 struct pids_cgroup *p;
130 for (p = pids; parent_pids(p); p = parent_pids(p))
131 atomic64_add(num, &p->counter);
135 * pids_try_charge - hierarchically try to charge the pid count
136 * @pids: the pid cgroup state
137 * @num: the number of pids to charge
139 * This function follows the set limit. It will fail if the charge would cause
140 * the new value to exceed the hierarchical limit. Returns 0 if the charge
141 * succeeded, otherwise -EAGAIN.
143 static int pids_try_charge(struct pids_cgroup *pids, int num)
145 struct pids_cgroup *p, *q;
147 for (p = pids; parent_pids(p); p = parent_pids(p)) {
148 int64_t new = atomic64_add_return(num, &p->counter);
151 * Since new is capped to the maximum number of pid_t, if
152 * p->limit is %PIDS_MAX then we know that this test will never
162 for (q = pids; q != p; q = parent_pids(q))
169 static int pids_can_attach(struct cgroup_taskset *tset)
171 struct task_struct *task;
172 struct cgroup_subsys_state *dst_css;
174 cgroup_taskset_for_each(task, dst_css, tset) {
175 struct pids_cgroup *pids = css_pids(dst_css);
176 struct cgroup_subsys_state *old_css;
177 struct pids_cgroup *old_pids;
180 * No need to pin @old_css between here and cancel_attach()
181 * because cgroup core protects it from being freed before
182 * the migration completes or fails.
184 old_css = task_css(task, pids_cgrp_id);
185 old_pids = css_pids(old_css);
187 pids_charge(pids, 1);
188 pids_uncharge(old_pids, 1);
194 static void pids_cancel_attach(struct cgroup_taskset *tset)
196 struct task_struct *task;
197 struct cgroup_subsys_state *dst_css;
199 cgroup_taskset_for_each(task, dst_css, tset) {
200 struct pids_cgroup *pids = css_pids(dst_css);
201 struct cgroup_subsys_state *old_css;
202 struct pids_cgroup *old_pids;
204 old_css = task_css(task, pids_cgrp_id);
205 old_pids = css_pids(old_css);
207 pids_charge(old_pids, 1);
208 pids_uncharge(pids, 1);
213 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
214 * on cgroup_threadgroup_change_begin() held by the copy_process().
216 static int pids_can_fork(struct task_struct *task)
218 struct cgroup_subsys_state *css;
219 struct pids_cgroup *pids;
222 css = task_css_check(current, pids_cgrp_id, true);
223 pids = css_pids(css);
224 err = pids_try_charge(pids, 1);
226 /* Only log the first time events_limit is incremented. */
227 if (atomic64_inc_return(&pids->events_limit) == 1) {
228 pr_info("cgroup: fork rejected by pids controller in ");
229 pr_cont_cgroup_path(css->cgroup);
232 cgroup_file_notify(&pids->events_file);
237 static void pids_cancel_fork(struct task_struct *task)
239 struct cgroup_subsys_state *css;
240 struct pids_cgroup *pids;
242 css = task_css_check(current, pids_cgrp_id, true);
243 pids = css_pids(css);
244 pids_uncharge(pids, 1);
247 static void pids_release(struct task_struct *task)
249 struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
251 pids_uncharge(pids, 1);
254 static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
255 size_t nbytes, loff_t off)
257 struct cgroup_subsys_state *css = of_css(of);
258 struct pids_cgroup *pids = css_pids(css);
263 if (!strcmp(buf, PIDS_MAX_STR)) {
268 err = kstrtoll(buf, 0, &limit);
272 if (limit < 0 || limit >= PIDS_MAX)
277 * Limit updates don't need to be mutex'd, since it isn't
278 * critical that any racing fork()s follow the new limit.
284 static int pids_max_show(struct seq_file *sf, void *v)
286 struct cgroup_subsys_state *css = seq_css(sf);
287 struct pids_cgroup *pids = css_pids(css);
288 int64_t limit = pids->limit;
290 if (limit >= PIDS_MAX)
291 seq_printf(sf, "%s\n", PIDS_MAX_STR);
293 seq_printf(sf, "%lld\n", limit);
298 static s64 pids_current_read(struct cgroup_subsys_state *css,
301 struct pids_cgroup *pids = css_pids(css);
303 return atomic64_read(&pids->counter);
306 static int pids_events_show(struct seq_file *sf, void *v)
308 struct pids_cgroup *pids = css_pids(seq_css(sf));
310 seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
314 static struct cftype pids_files[] = {
317 .write = pids_max_write,
318 .seq_show = pids_max_show,
319 .flags = CFTYPE_NOT_ON_ROOT,
323 .read_s64 = pids_current_read,
324 .flags = CFTYPE_NOT_ON_ROOT,
328 .seq_show = pids_events_show,
329 .file_offset = offsetof(struct pids_cgroup, events_file),
330 .flags = CFTYPE_NOT_ON_ROOT,
335 struct cgroup_subsys pids_cgrp_subsys = {
336 .css_alloc = pids_css_alloc,
337 .css_free = pids_css_free,
338 .can_attach = pids_can_attach,
339 .cancel_attach = pids_cancel_attach,
340 .can_fork = pids_can_fork,
341 .cancel_fork = pids_cancel_fork,
342 .release = pids_release,
343 .legacy_cftypes = pids_files,
344 .dfl_cftypes = pids_files,