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Linux/kernel/cgroup_pids.c

  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 
 43 struct 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 
 60 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
 61 {
 62         return container_of(css, struct pids_cgroup, css);
 63 }
 64 
 65 static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
 66 {
 67         return css_pids(pids->css.parent);
 68 }
 69 
 70 static struct cgroup_subsys_state *
 71 pids_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 
 85 static 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  */
 98 static 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  */
112 static 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  */
129 static 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  */
146 static 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 
164 revert:
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 
172 static 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 
197 static 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 threadgroup_change_begin() held by the copy_process().
218  */
219 static 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(task_cgroup(current, pids_cgrp_id));
233                         pr_cont("\n");
234                 }
235                 cgroup_file_notify(&pids->events_file);
236         }
237         return err;
238 }
239 
240 static 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 
250 static 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 
257 static 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 
278 set_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 
287 static 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 
301 static 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 
309 static 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 
317 static 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 
338 struct 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 };
349 

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