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(css->cgroup);
 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 };