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};