1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Process number limiting controller for cgroups.
4 *
5 * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
6 * after a certain limit is reached.
7 *
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.
12 *
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.
20 *
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).
24 *
25 * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
26 * a superset of parent/child/pids.current.
27 *
28 * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
29 */
30
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>
36#include <linux/sched/task.h>
37
38#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
39#define PIDS_MAX_STR "max"
40
41struct pids_cgroup {
42 struct cgroup_subsys_state css;
43
44 /*
45 * Use 64-bit types so that we can safely represent "max" as
46 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
47 */
48 atomic64_t counter;
49 atomic64_t limit;
50 int64_t watermark;
51
52 /* Handle for "pids.events" */
53 struct cgroup_file events_file;
54
55 /* Number of times fork failed because limit was hit. */
56 atomic64_t events_limit;
57};
58
59static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
60{
61 return container_of(css, struct pids_cgroup, css);
62}
63
64static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
65{
66 return css_pids(css: pids->css.parent);
67}
68
69static struct cgroup_subsys_state *
70pids_css_alloc(struct cgroup_subsys_state *parent)
71{
72 struct pids_cgroup *pids;
73
74 pids = kzalloc(size: sizeof(struct pids_cgroup), GFP_KERNEL);
75 if (!pids)
76 return ERR_PTR(error: -ENOMEM);
77
78 atomic64_set(v: &pids->counter, i: 0);
79 atomic64_set(v: &pids->limit, PIDS_MAX);
80 atomic64_set(v: &pids->events_limit, i: 0);
81 return &pids->css;
82}
83
84static void pids_css_free(struct cgroup_subsys_state *css)
85{
86 kfree(objp: css_pids(css));
87}
88
89static void pids_update_watermark(struct pids_cgroup *p, int64_t nr_pids)
90{
91 /*
92 * This is racy, but we don't need perfectly accurate tallying of
93 * the watermark, and this lets us avoid extra atomic overhead.
94 */
95 if (nr_pids > READ_ONCE(p->watermark))
96 WRITE_ONCE(p->watermark, nr_pids);
97}
98
99/**
100 * pids_cancel - uncharge the local pid count
101 * @pids: the pid cgroup state
102 * @num: the number of pids to cancel
103 *
104 * This function will WARN if the pid count goes under 0, because such a case is
105 * a bug in the pids controller proper.
106 */
107static void pids_cancel(struct pids_cgroup *pids, int num)
108{
109 /*
110 * A negative count (or overflow for that matter) is invalid,
111 * and indicates a bug in the `pids` controller proper.
112 */
113 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
114}
115
116/**
117 * pids_uncharge - hierarchically uncharge the pid count
118 * @pids: the pid cgroup state
119 * @num: the number of pids to uncharge
120 */
121static void pids_uncharge(struct pids_cgroup *pids, int num)
122{
123 struct pids_cgroup *p;
124
125 for (p = pids; parent_pids(pids: p); p = parent_pids(pids: p))
126 pids_cancel(pids: p, num);
127}
128
129/**
130 * pids_charge - hierarchically charge the pid count
131 * @pids: the pid cgroup state
132 * @num: the number of pids to charge
133 *
134 * This function does *not* follow the pid limit set. It cannot fail and the new
135 * pid count may exceed the limit. This is only used for reverting failed
136 * attaches, where there is no other way out than violating the limit.
137 */
138static void pids_charge(struct pids_cgroup *pids, int num)
139{
140 struct pids_cgroup *p;
141
142 for (p = pids; parent_pids(pids: p); p = parent_pids(pids: p)) {
143 int64_t new = atomic64_add_return(i: num, v: &p->counter);
144
145 pids_update_watermark(p, nr_pids: new);
146 }
147}
148
149/**
150 * pids_try_charge - hierarchically try to charge the pid count
151 * @pids: the pid cgroup state
152 * @num: the number of pids to charge
153 *
154 * This function follows the set limit. It will fail if the charge would cause
155 * the new value to exceed the hierarchical limit. Returns 0 if the charge
156 * succeeded, otherwise -EAGAIN.
157 */
158static int pids_try_charge(struct pids_cgroup *pids, int num)
159{
160 struct pids_cgroup *p, *q;
161
162 for (p = pids; parent_pids(pids: p); p = parent_pids(pids: p)) {
163 int64_t new = atomic64_add_return(i: num, v: &p->counter);
164 int64_t limit = atomic64_read(v: &p->limit);
165
166 /*
167 * Since new is capped to the maximum number of pid_t, if
168 * p->limit is %PIDS_MAX then we know that this test will never
169 * fail.
170 */
171 if (new > limit)
172 goto revert;
173
174 /*
175 * Not technically accurate if we go over limit somewhere up
176 * the hierarchy, but that's tolerable for the watermark.
177 */
178 pids_update_watermark(p, nr_pids: new);
179 }
180
181 return 0;
182
183revert:
184 for (q = pids; q != p; q = parent_pids(pids: q))
185 pids_cancel(pids: q, num);
186 pids_cancel(pids: p, num);
187
188 return -EAGAIN;
189}
190
191static int pids_can_attach(struct cgroup_taskset *tset)
192{
193 struct task_struct *task;
194 struct cgroup_subsys_state *dst_css;
195
196 cgroup_taskset_for_each(task, dst_css, tset) {
197 struct pids_cgroup *pids = css_pids(css: dst_css);
198 struct cgroup_subsys_state *old_css;
199 struct pids_cgroup *old_pids;
200
201 /*
202 * No need to pin @old_css between here and cancel_attach()
203 * because cgroup core protects it from being freed before
204 * the migration completes or fails.
205 */
206 old_css = task_css(task, subsys_id: pids_cgrp_id);
207 old_pids = css_pids(css: old_css);
208
209 pids_charge(pids, num: 1);
210 pids_uncharge(pids: old_pids, num: 1);
211 }
212
213 return 0;
214}
215
216static void pids_cancel_attach(struct cgroup_taskset *tset)
217{
218 struct task_struct *task;
219 struct cgroup_subsys_state *dst_css;
220
221 cgroup_taskset_for_each(task, dst_css, tset) {
222 struct pids_cgroup *pids = css_pids(css: dst_css);
223 struct cgroup_subsys_state *old_css;
224 struct pids_cgroup *old_pids;
225
226 old_css = task_css(task, subsys_id: pids_cgrp_id);
227 old_pids = css_pids(css: old_css);
228
229 pids_charge(pids: old_pids, num: 1);
230 pids_uncharge(pids, num: 1);
231 }
232}
233
234/*
235 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
236 * on cgroup_threadgroup_change_begin() held by the copy_process().
237 */
238static int pids_can_fork(struct task_struct *task, struct css_set *cset)
239{
240 struct cgroup_subsys_state *css;
241 struct pids_cgroup *pids;
242 int err;
243
244 if (cset)
245 css = cset->subsys[pids_cgrp_id];
246 else
247 css = task_css_check(current, pids_cgrp_id, true);
248 pids = css_pids(css);
249 err = pids_try_charge(pids, num: 1);
250 if (err) {
251 /* Only log the first time events_limit is incremented. */
252 if (atomic64_inc_return(v: &pids->events_limit) == 1) {
253 pr_info("cgroup: fork rejected by pids controller in ");
254 pr_cont_cgroup_path(cgrp: css->cgroup);
255 pr_cont("\n");
256 }
257 cgroup_file_notify(cfile: &pids->events_file);
258 }
259 return err;
260}
261
262static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
263{
264 struct cgroup_subsys_state *css;
265 struct pids_cgroup *pids;
266
267 if (cset)
268 css = cset->subsys[pids_cgrp_id];
269 else
270 css = task_css_check(current, pids_cgrp_id, true);
271 pids = css_pids(css);
272 pids_uncharge(pids, num: 1);
273}
274
275static void pids_release(struct task_struct *task)
276{
277 struct pids_cgroup *pids = css_pids(css: task_css(task, subsys_id: pids_cgrp_id));
278
279 pids_uncharge(pids, num: 1);
280}
281
282static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
283 size_t nbytes, loff_t off)
284{
285 struct cgroup_subsys_state *css = of_css(of);
286 struct pids_cgroup *pids = css_pids(css);
287 int64_t limit;
288 int err;
289
290 buf = strstrip(str: buf);
291 if (!strcmp(buf, PIDS_MAX_STR)) {
292 limit = PIDS_MAX;
293 goto set_limit;
294 }
295
296 err = kstrtoll(s: buf, base: 0, res: &limit);
297 if (err)
298 return err;
299
300 if (limit < 0 || limit >= PIDS_MAX)
301 return -EINVAL;
302
303set_limit:
304 /*
305 * Limit updates don't need to be mutex'd, since it isn't
306 * critical that any racing fork()s follow the new limit.
307 */
308 atomic64_set(v: &pids->limit, i: limit);
309 return nbytes;
310}
311
312static int pids_max_show(struct seq_file *sf, void *v)
313{
314 struct cgroup_subsys_state *css = seq_css(seq: sf);
315 struct pids_cgroup *pids = css_pids(css);
316 int64_t limit = atomic64_read(v: &pids->limit);
317
318 if (limit >= PIDS_MAX)
319 seq_printf(m: sf, fmt: "%s\n", PIDS_MAX_STR);
320 else
321 seq_printf(m: sf, fmt: "%lld\n", limit);
322
323 return 0;
324}
325
326static s64 pids_current_read(struct cgroup_subsys_state *css,
327 struct cftype *cft)
328{
329 struct pids_cgroup *pids = css_pids(css);
330
331 return atomic64_read(v: &pids->counter);
332}
333
334static s64 pids_peak_read(struct cgroup_subsys_state *css,
335 struct cftype *cft)
336{
337 struct pids_cgroup *pids = css_pids(css);
338
339 return READ_ONCE(pids->watermark);
340}
341
342static int pids_events_show(struct seq_file *sf, void *v)
343{
344 struct pids_cgroup *pids = css_pids(css: seq_css(seq: sf));
345
346 seq_printf(m: sf, fmt: "max %lld\n", (s64)atomic64_read(v: &pids->events_limit));
347 return 0;
348}
349
350static struct cftype pids_files[] = {
351 {
352 .name = "max",
353 .write = pids_max_write,
354 .seq_show = pids_max_show,
355 .flags = CFTYPE_NOT_ON_ROOT,
356 },
357 {
358 .name = "current",
359 .read_s64 = pids_current_read,
360 .flags = CFTYPE_NOT_ON_ROOT,
361 },
362 {
363 .name = "peak",
364 .flags = CFTYPE_NOT_ON_ROOT,
365 .read_s64 = pids_peak_read,
366 },
367 {
368 .name = "events",
369 .seq_show = pids_events_show,
370 .file_offset = offsetof(struct pids_cgroup, events_file),
371 .flags = CFTYPE_NOT_ON_ROOT,
372 },
373 { } /* terminate */
374};
375
376struct cgroup_subsys pids_cgrp_subsys = {
377 .css_alloc = pids_css_alloc,
378 .css_free = pids_css_free,
379 .can_attach = pids_can_attach,
380 .cancel_attach = pids_cancel_attach,
381 .can_fork = pids_can_fork,
382 .cancel_fork = pids_cancel_fork,
383 .release = pids_release,
384 .legacy_cftypes = pids_files,
385 .dfl_cftypes = pids_files,
386 .threaded = true,
387};
388

source code of linux/kernel/cgroup/pids.c