Loading...
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
37#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
38#define PIDS_MAX_STR "max"
39
40struct pids_cgroup {
41 struct cgroup_subsys_state css;
42
43 /*
44 * Use 64-bit types so that we can safely represent "max" as
45 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
46 */
47 atomic64_t counter;
48 int64_t limit;
49
50 /* Handle for "pids.events" */
51 struct cgroup_file events_file;
52
53 /* Number of times fork failed because limit was hit. */
54 atomic64_t events_limit;
55};
56
57static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
58{
59 return container_of(css, struct pids_cgroup, css);
60}
61
62static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
63{
64 return css_pids(pids->css.parent);
65}
66
67static struct cgroup_subsys_state *
68pids_css_alloc(struct cgroup_subsys_state *parent)
69{
70 struct pids_cgroup *pids;
71
72 pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
73 if (!pids)
74 return ERR_PTR(-ENOMEM);
75
76 pids->limit = PIDS_MAX;
77 atomic64_set(&pids->counter, 0);
78 atomic64_set(&pids->events_limit, 0);
79 return &pids->css;
80}
81
82static void pids_css_free(struct cgroup_subsys_state *css)
83{
84 kfree(css_pids(css));
85}
86
87/**
88 * pids_cancel - uncharge the local pid count
89 * @pids: the pid cgroup state
90 * @num: the number of pids to cancel
91 *
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.
94 */
95static void pids_cancel(struct pids_cgroup *pids, int num)
96{
97 /*
98 * A negative count (or overflow for that matter) is invalid,
99 * and indicates a bug in the `pids` controller proper.
100 */
101 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
102}
103
104/**
105 * pids_uncharge - hierarchically uncharge the pid count
106 * @pids: the pid cgroup state
107 * @num: the number of pids to uncharge
108 */
109static void pids_uncharge(struct pids_cgroup *pids, int num)
110{
111 struct pids_cgroup *p;
112
113 for (p = pids; parent_pids(p); p = parent_pids(p))
114 pids_cancel(p, num);
115}
116
117/**
118 * pids_charge - hierarchically charge the pid count
119 * @pids: the pid cgroup state
120 * @num: the number of pids to charge
121 *
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.
125 */
126static void pids_charge(struct pids_cgroup *pids, int num)
127{
128 struct pids_cgroup *p;
129
130 for (p = pids; parent_pids(p); p = parent_pids(p))
131 atomic64_add(num, &p->counter);
132}
133
134/**
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
138 *
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.
142 */
143static int pids_try_charge(struct pids_cgroup *pids, int num)
144{
145 struct pids_cgroup *p, *q;
146
147 for (p = pids; parent_pids(p); p = parent_pids(p)) {
148 int64_t new = atomic64_add_return(num, &p->counter);
149
150 /*
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
153 * fail.
154 */
155 if (new > p->limit)
156 goto revert;
157 }
158
159 return 0;
160
161revert:
162 for (q = pids; q != p; q = parent_pids(q))
163 pids_cancel(q, num);
164 pids_cancel(p, num);
165
166 return -EAGAIN;
167}
168
169static int pids_can_attach(struct cgroup_taskset *tset)
170{
171 struct task_struct *task;
172 struct cgroup_subsys_state *dst_css;
173
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;
178
179 /*
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.
183 */
184 old_css = task_css(task, pids_cgrp_id);
185 old_pids = css_pids(old_css);
186
187 pids_charge(pids, 1);
188 pids_uncharge(old_pids, 1);
189 }
190
191 return 0;
192}
193
194static void pids_cancel_attach(struct cgroup_taskset *tset)
195{
196 struct task_struct *task;
197 struct cgroup_subsys_state *dst_css;
198
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;
203
204 old_css = task_css(task, pids_cgrp_id);
205 old_pids = css_pids(old_css);
206
207 pids_charge(old_pids, 1);
208 pids_uncharge(pids, 1);
209 }
210}
211
212/*
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().
215 */
216static int pids_can_fork(struct task_struct *task)
217{
218 struct cgroup_subsys_state *css;
219 struct pids_cgroup *pids;
220 int err;
221
222 css = task_css_check(current, pids_cgrp_id, true);
223 pids = css_pids(css);
224 err = pids_try_charge(pids, 1);
225 if (err) {
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);
230 pr_cont("\n");
231 }
232 cgroup_file_notify(&pids->events_file);
233 }
234 return err;
235}
236
237static void pids_cancel_fork(struct task_struct *task)
238{
239 struct cgroup_subsys_state *css;
240 struct pids_cgroup *pids;
241
242 css = task_css_check(current, pids_cgrp_id, true);
243 pids = css_pids(css);
244 pids_uncharge(pids, 1);
245}
246
247static void pids_release(struct task_struct *task)
248{
249 struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
250
251 pids_uncharge(pids, 1);
252}
253
254static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
255 size_t nbytes, loff_t off)
256{
257 struct cgroup_subsys_state *css = of_css(of);
258 struct pids_cgroup *pids = css_pids(css);
259 int64_t limit;
260 int err;
261
262 buf = strstrip(buf);
263 if (!strcmp(buf, PIDS_MAX_STR)) {
264 limit = PIDS_MAX;
265 goto set_limit;
266 }
267
268 err = kstrtoll(buf, 0, &limit);
269 if (err)
270 return err;
271
272 if (limit < 0 || limit >= PIDS_MAX)
273 return -EINVAL;
274
275set_limit:
276 /*
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.
279 */
280 pids->limit = limit;
281 return nbytes;
282}
283
284static int pids_max_show(struct seq_file *sf, void *v)
285{
286 struct cgroup_subsys_state *css = seq_css(sf);
287 struct pids_cgroup *pids = css_pids(css);
288 int64_t limit = pids->limit;
289
290 if (limit >= PIDS_MAX)
291 seq_printf(sf, "%s\n", PIDS_MAX_STR);
292 else
293 seq_printf(sf, "%lld\n", limit);
294
295 return 0;
296}
297
298static s64 pids_current_read(struct cgroup_subsys_state *css,
299 struct cftype *cft)
300{
301 struct pids_cgroup *pids = css_pids(css);
302
303 return atomic64_read(&pids->counter);
304}
305
306static int pids_events_show(struct seq_file *sf, void *v)
307{
308 struct pids_cgroup *pids = css_pids(seq_css(sf));
309
310 seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
311 return 0;
312}
313
314static struct cftype pids_files[] = {
315 {
316 .name = "max",
317 .write = pids_max_write,
318 .seq_show = pids_max_show,
319 .flags = CFTYPE_NOT_ON_ROOT,
320 },
321 {
322 .name = "current",
323 .read_s64 = pids_current_read,
324 .flags = CFTYPE_NOT_ON_ROOT,
325 },
326 {
327 .name = "events",
328 .seq_show = pids_events_show,
329 .file_offset = offsetof(struct pids_cgroup, events_file),
330 .flags = CFTYPE_NOT_ON_ROOT,
331 },
332 { } /* terminate */
333};
334
335struct 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,
345 .threaded = true,
346};
1/*
2 * Process number limiting controller for cgroups.
3 *
4 * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
5 * after a certain limit is reached.
6 *
7 * Since it is trivial to hit the task limit without hitting any kmemcg limits
8 * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
9 * preventable in the scope of a cgroup hierarchy by allowing resource limiting
10 * of the number of tasks in a cgroup.
11 *
12 * In order to use the `pids` controller, set the maximum number of tasks in
13 * pids.max (this is not available in the root cgroup for obvious reasons). The
14 * number of processes currently in the cgroup is given by pids.current.
15 * Organisational operations are not blocked by cgroup policies, so it is
16 * possible to have pids.current > pids.max. However, it is not possible to
17 * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
18 * would cause a cgroup policy to be violated.
19 *
20 * To set a cgroup to have no limit, set pids.max to "max". This is the default
21 * for all new cgroups (N.B. that PID limits are hierarchical, so the most
22 * stringent limit in the hierarchy is followed).
23 *
24 * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
25 * a superset of parent/child/pids.current.
26 *
27 * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
28 *
29 * This file is subject to the terms and conditions of version 2 of the GNU
30 * General Public License. See the file COPYING in the main directory of the
31 * Linux distribution for more details.
32 */
33
34#include <linux/kernel.h>
35#include <linux/threads.h>
36#include <linux/atomic.h>
37#include <linux/cgroup.h>
38#include <linux/slab.h>
39
40#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
41#define PIDS_MAX_STR "max"
42
43struct pids_cgroup {
44 struct cgroup_subsys_state css;
45
46 /*
47 * Use 64-bit types so that we can safely represent "max" as
48 * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
49 */
50 atomic64_t counter;
51 int64_t limit;
52
53 /* Handle for "pids.events" */
54 struct cgroup_file events_file;
55
56 /* Number of times fork failed because limit was hit. */
57 atomic64_t events_limit;
58};
59
60static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
61{
62 return container_of(css, struct pids_cgroup, css);
63}
64
65static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
66{
67 return css_pids(pids->css.parent);
68}
69
70static struct cgroup_subsys_state *
71pids_css_alloc(struct cgroup_subsys_state *parent)
72{
73 struct pids_cgroup *pids;
74
75 pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
76 if (!pids)
77 return ERR_PTR(-ENOMEM);
78
79 pids->limit = PIDS_MAX;
80 atomic64_set(&pids->counter, 0);
81 atomic64_set(&pids->events_limit, 0);
82 return &pids->css;
83}
84
85static void pids_css_free(struct cgroup_subsys_state *css)
86{
87 kfree(css_pids(css));
88}
89
90/**
91 * pids_cancel - uncharge the local pid count
92 * @pids: the pid cgroup state
93 * @num: the number of pids to cancel
94 *
95 * This function will WARN if the pid count goes under 0, because such a case is
96 * a bug in the pids controller proper.
97 */
98static void pids_cancel(struct pids_cgroup *pids, int num)
99{
100 /*
101 * A negative count (or overflow for that matter) is invalid,
102 * and indicates a bug in the `pids` controller proper.
103 */
104 WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
105}
106
107/**
108 * pids_uncharge - hierarchically uncharge the pid count
109 * @pids: the pid cgroup state
110 * @num: the number of pids to uncharge
111 */
112static void pids_uncharge(struct pids_cgroup *pids, int num)
113{
114 struct pids_cgroup *p;
115
116 for (p = pids; parent_pids(p); p = parent_pids(p))
117 pids_cancel(p, num);
118}
119
120/**
121 * pids_charge - hierarchically charge the pid count
122 * @pids: the pid cgroup state
123 * @num: the number of pids to charge
124 *
125 * This function does *not* follow the pid limit set. It cannot fail and the new
126 * pid count may exceed the limit. This is only used for reverting failed
127 * attaches, where there is no other way out than violating the limit.
128 */
129static void pids_charge(struct pids_cgroup *pids, int num)
130{
131 struct pids_cgroup *p;
132
133 for (p = pids; parent_pids(p); p = parent_pids(p))
134 atomic64_add(num, &p->counter);
135}
136
137/**
138 * pids_try_charge - hierarchically try to charge the pid count
139 * @pids: the pid cgroup state
140 * @num: the number of pids to charge
141 *
142 * This function follows the set limit. It will fail if the charge would cause
143 * the new value to exceed the hierarchical limit. Returns 0 if the charge
144 * succeeded, otherwise -EAGAIN.
145 */
146static int pids_try_charge(struct pids_cgroup *pids, int num)
147{
148 struct pids_cgroup *p, *q;
149
150 for (p = pids; parent_pids(p); p = parent_pids(p)) {
151 int64_t new = atomic64_add_return(num, &p->counter);
152
153 /*
154 * Since new is capped to the maximum number of pid_t, if
155 * p->limit is %PIDS_MAX then we know that this test will never
156 * fail.
157 */
158 if (new > p->limit)
159 goto revert;
160 }
161
162 return 0;
163
164revert:
165 for (q = pids; q != p; q = parent_pids(q))
166 pids_cancel(q, num);
167 pids_cancel(p, num);
168
169 return -EAGAIN;
170}
171
172static int pids_can_attach(struct cgroup_taskset *tset)
173{
174 struct task_struct *task;
175 struct cgroup_subsys_state *dst_css;
176
177 cgroup_taskset_for_each(task, dst_css, tset) {
178 struct pids_cgroup *pids = css_pids(dst_css);
179 struct cgroup_subsys_state *old_css;
180 struct pids_cgroup *old_pids;
181
182 /*
183 * No need to pin @old_css between here and cancel_attach()
184 * because cgroup core protects it from being freed before
185 * the migration completes or fails.
186 */
187 old_css = task_css(task, pids_cgrp_id);
188 old_pids = css_pids(old_css);
189
190 pids_charge(pids, 1);
191 pids_uncharge(old_pids, 1);
192 }
193
194 return 0;
195}
196
197static void pids_cancel_attach(struct cgroup_taskset *tset)
198{
199 struct task_struct *task;
200 struct cgroup_subsys_state *dst_css;
201
202 cgroup_taskset_for_each(task, dst_css, tset) {
203 struct pids_cgroup *pids = css_pids(dst_css);
204 struct cgroup_subsys_state *old_css;
205 struct pids_cgroup *old_pids;
206
207 old_css = task_css(task, pids_cgrp_id);
208 old_pids = css_pids(old_css);
209
210 pids_charge(old_pids, 1);
211 pids_uncharge(pids, 1);
212 }
213}
214
215/*
216 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
217 * on cgroup_threadgroup_change_begin() held by the copy_process().
218 */
219static int pids_can_fork(struct task_struct *task)
220{
221 struct cgroup_subsys_state *css;
222 struct pids_cgroup *pids;
223 int err;
224
225 css = task_css_check(current, pids_cgrp_id, true);
226 pids = css_pids(css);
227 err = pids_try_charge(pids, 1);
228 if (err) {
229 /* Only log the first time events_limit is incremented. */
230 if (atomic64_inc_return(&pids->events_limit) == 1) {
231 pr_info("cgroup: fork rejected by pids controller in ");
232 pr_cont_cgroup_path(css->cgroup);
233 pr_cont("\n");
234 }
235 cgroup_file_notify(&pids->events_file);
236 }
237 return err;
238}
239
240static void pids_cancel_fork(struct task_struct *task)
241{
242 struct cgroup_subsys_state *css;
243 struct pids_cgroup *pids;
244
245 css = task_css_check(current, pids_cgrp_id, true);
246 pids = css_pids(css);
247 pids_uncharge(pids, 1);
248}
249
250static void pids_free(struct task_struct *task)
251{
252 struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
253
254 pids_uncharge(pids, 1);
255}
256
257static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
258 size_t nbytes, loff_t off)
259{
260 struct cgroup_subsys_state *css = of_css(of);
261 struct pids_cgroup *pids = css_pids(css);
262 int64_t limit;
263 int err;
264
265 buf = strstrip(buf);
266 if (!strcmp(buf, PIDS_MAX_STR)) {
267 limit = PIDS_MAX;
268 goto set_limit;
269 }
270
271 err = kstrtoll(buf, 0, &limit);
272 if (err)
273 return err;
274
275 if (limit < 0 || limit >= PIDS_MAX)
276 return -EINVAL;
277
278set_limit:
279 /*
280 * Limit updates don't need to be mutex'd, since it isn't
281 * critical that any racing fork()s follow the new limit.
282 */
283 pids->limit = limit;
284 return nbytes;
285}
286
287static int pids_max_show(struct seq_file *sf, void *v)
288{
289 struct cgroup_subsys_state *css = seq_css(sf);
290 struct pids_cgroup *pids = css_pids(css);
291 int64_t limit = pids->limit;
292
293 if (limit >= PIDS_MAX)
294 seq_printf(sf, "%s\n", PIDS_MAX_STR);
295 else
296 seq_printf(sf, "%lld\n", limit);
297
298 return 0;
299}
300
301static s64 pids_current_read(struct cgroup_subsys_state *css,
302 struct cftype *cft)
303{
304 struct pids_cgroup *pids = css_pids(css);
305
306 return atomic64_read(&pids->counter);
307}
308
309static int pids_events_show(struct seq_file *sf, void *v)
310{
311 struct pids_cgroup *pids = css_pids(seq_css(sf));
312
313 seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
314 return 0;
315}
316
317static struct cftype pids_files[] = {
318 {
319 .name = "max",
320 .write = pids_max_write,
321 .seq_show = pids_max_show,
322 .flags = CFTYPE_NOT_ON_ROOT,
323 },
324 {
325 .name = "current",
326 .read_s64 = pids_current_read,
327 .flags = CFTYPE_NOT_ON_ROOT,
328 },
329 {
330 .name = "events",
331 .seq_show = pids_events_show,
332 .file_offset = offsetof(struct pids_cgroup, events_file),
333 .flags = CFTYPE_NOT_ON_ROOT,
334 },
335 { } /* terminate */
336};
337
338struct cgroup_subsys pids_cgrp_subsys = {
339 .css_alloc = pids_css_alloc,
340 .css_free = pids_css_free,
341 .can_attach = pids_can_attach,
342 .cancel_attach = pids_cancel_attach,
343 .can_fork = pids_can_fork,
344 .cancel_fork = pids_cancel_fork,
345 .free = pids_free,
346 .legacy_cftypes = pids_files,
347 .dfl_cftypes = pids_files,
348 .threaded = true,
349};