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
 54static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
 55{
 56	return container_of(css, struct pids_cgroup, css);
 57}
 58
 59static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
 60{
 61	return css_pids(pids->css.parent);
 62}
 63
 64static struct cgroup_subsys_state *
 65pids_css_alloc(struct cgroup_subsys_state *parent)
 66{
 67	struct pids_cgroup *pids;
 68
 69	pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
 70	if (!pids)
 71		return ERR_PTR(-ENOMEM);
 72
 73	pids->limit = PIDS_MAX;
 74	atomic64_set(&pids->counter, 0);
 75	return &pids->css;
 76}
 77
 78static void pids_css_free(struct cgroup_subsys_state *css)
 79{
 80	kfree(css_pids(css));
 81}
 82
 83/**
 84 * pids_cancel - uncharge the local pid count
 85 * @pids: the pid cgroup state
 86 * @num: the number of pids to cancel
 87 *
 88 * This function will WARN if the pid count goes under 0, because such a case is
 89 * a bug in the pids controller proper.
 90 */
 91static void pids_cancel(struct pids_cgroup *pids, int num)
 92{
 93	/*
 94	 * A negative count (or overflow for that matter) is invalid,
 95	 * and indicates a bug in the `pids` controller proper.
 96	 */
 97	WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
 98}
 99
100/**
101 * pids_uncharge - hierarchically uncharge the pid count
102 * @pids: the pid cgroup state
103 * @num: the number of pids to uncharge
104 */
105static void pids_uncharge(struct pids_cgroup *pids, int num)
106{
107	struct pids_cgroup *p;
108
109	for (p = pids; parent_pids(p); p = parent_pids(p))
110		pids_cancel(p, num);
111}
112
113/**
114 * pids_charge - hierarchically charge the pid count
115 * @pids: the pid cgroup state
116 * @num: the number of pids to charge
117 *
118 * This function does *not* follow the pid limit set. It cannot fail and the new
119 * pid count may exceed the limit. This is only used for reverting failed
120 * attaches, where there is no other way out than violating the limit.
121 */
122static void pids_charge(struct pids_cgroup *pids, int num)
123{
124	struct pids_cgroup *p;
125
126	for (p = pids; parent_pids(p); p = parent_pids(p))
127		atomic64_add(num, &p->counter);
128}
129
130/**
131 * pids_try_charge - hierarchically try to charge the pid count
132 * @pids: the pid cgroup state
133 * @num: the number of pids to charge
134 *
135 * This function follows the set limit. It will fail if the charge would cause
136 * the new value to exceed the hierarchical limit. Returns 0 if the charge
137 * succeeded, otherwise -EAGAIN.
138 */
139static int pids_try_charge(struct pids_cgroup *pids, int num)
140{
141	struct pids_cgroup *p, *q;
142
143	for (p = pids; parent_pids(p); p = parent_pids(p)) {
144		int64_t new = atomic64_add_return(num, &p->counter);
145
146		/*
147		 * Since new is capped to the maximum number of pid_t, if
148		 * p->limit is %PIDS_MAX then we know that this test will never
149		 * fail.
150		 */
151		if (new > p->limit)
152			goto revert;
153	}
154
155	return 0;
156
157revert:
158	for (q = pids; q != p; q = parent_pids(q))
159		pids_cancel(q, num);
160	pids_cancel(p, num);
161
162	return -EAGAIN;
163}
164
165static int pids_can_attach(struct cgroup_taskset *tset)
166{
167	struct task_struct *task;
168	struct cgroup_subsys_state *dst_css;
169
170	cgroup_taskset_for_each(task, dst_css, tset) {
171		struct pids_cgroup *pids = css_pids(dst_css);
172		struct cgroup_subsys_state *old_css;
173		struct pids_cgroup *old_pids;
174
175		/*
176		 * No need to pin @old_css between here and cancel_attach()
177		 * because cgroup core protects it from being freed before
178		 * the migration completes or fails.
179		 */
180		old_css = task_css(task, pids_cgrp_id);
181		old_pids = css_pids(old_css);
182
183		pids_charge(pids, 1);
184		pids_uncharge(old_pids, 1);
185	}
186
187	return 0;
188}
189
190static void pids_cancel_attach(struct cgroup_taskset *tset)
191{
192	struct task_struct *task;
193	struct cgroup_subsys_state *dst_css;
194
195	cgroup_taskset_for_each(task, dst_css, tset) {
196		struct pids_cgroup *pids = css_pids(dst_css);
197		struct cgroup_subsys_state *old_css;
198		struct pids_cgroup *old_pids;
199
200		old_css = task_css(task, pids_cgrp_id);
201		old_pids = css_pids(old_css);
202
203		pids_charge(old_pids, 1);
204		pids_uncharge(pids, 1);
205	}
206}
207
208/*
209 * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
210 * on threadgroup_change_begin() held by the copy_process().
211 */
212static int pids_can_fork(struct task_struct *task)
213{
214	struct cgroup_subsys_state *css;
215	struct pids_cgroup *pids;
216
217	css = task_css_check(current, pids_cgrp_id, true);
218	pids = css_pids(css);
219	return pids_try_charge(pids, 1);
220}
221
222static void pids_cancel_fork(struct task_struct *task)
223{
224	struct cgroup_subsys_state *css;
225	struct pids_cgroup *pids;
226
227	css = task_css_check(current, pids_cgrp_id, true);
228	pids = css_pids(css);
229	pids_uncharge(pids, 1);
230}
231
232static void pids_free(struct task_struct *task)
233{
234	struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
235
236	pids_uncharge(pids, 1);
237}
238
239static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
240			      size_t nbytes, loff_t off)
241{
242	struct cgroup_subsys_state *css = of_css(of);
243	struct pids_cgroup *pids = css_pids(css);
244	int64_t limit;
245	int err;
246
247	buf = strstrip(buf);
248	if (!strcmp(buf, PIDS_MAX_STR)) {
249		limit = PIDS_MAX;
250		goto set_limit;
251	}
252
253	err = kstrtoll(buf, 0, &limit);
254	if (err)
255		return err;
256
257	if (limit < 0 || limit >= PIDS_MAX)
258		return -EINVAL;
259
260set_limit:
261	/*
262	 * Limit updates don't need to be mutex'd, since it isn't
263	 * critical that any racing fork()s follow the new limit.
264	 */
265	pids->limit = limit;
266	return nbytes;
267}
268
269static int pids_max_show(struct seq_file *sf, void *v)
270{
271	struct cgroup_subsys_state *css = seq_css(sf);
272	struct pids_cgroup *pids = css_pids(css);
273	int64_t limit = pids->limit;
274
275	if (limit >= PIDS_MAX)
276		seq_printf(sf, "%s\n", PIDS_MAX_STR);
277	else
278		seq_printf(sf, "%lld\n", limit);
279
280	return 0;
281}
282
283static s64 pids_current_read(struct cgroup_subsys_state *css,
284			     struct cftype *cft)
285{
286	struct pids_cgroup *pids = css_pids(css);
287
288	return atomic64_read(&pids->counter);
289}
290
291static struct cftype pids_files[] = {
292	{
293		.name = "max",
294		.write = pids_max_write,
295		.seq_show = pids_max_show,
296		.flags = CFTYPE_NOT_ON_ROOT,
297	},
298	{
299		.name = "current",
300		.read_s64 = pids_current_read,
301		.flags = CFTYPE_NOT_ON_ROOT,
302	},
303	{ }	/* terminate */
304};
305
306struct cgroup_subsys pids_cgrp_subsys = {
307	.css_alloc	= pids_css_alloc,
308	.css_free	= pids_css_free,
309	.can_attach 	= pids_can_attach,
310	.cancel_attach 	= pids_cancel_attach,
311	.can_fork	= pids_can_fork,
312	.cancel_fork	= pids_cancel_fork,
313	.free		= pids_free,
314	.legacy_cftypes	= pids_files,
315	.dfl_cftypes	= pids_files,
316};