kernel/cgroup/pids.c

   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 #include <linux/sched/task.h>
  37
  38 #define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
  39 #define PIDS_MAX_STR "max"
  40
  41 struct pids_cgroup {
  42         struct cgroup_subsys_state      css;
  43
  44         /*
  45          * Use 64-bit types so that we can safely represent "max" as
  46          * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
  47          */
  48         atomic64_t                      counter;
  49         atomic64_t                      limit;
  50
  51         /* Handle for "pids.events" */
  52         struct cgroup_file              events_file;
  53
  54         /* Number of times fork failed because limit was hit. */
  55         atomic64_t                      events_limit;
  56 };
  57
  58 static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
  59 {
  60         return container_of(css, struct pids_cgroup, css);
  61 }
  62
  63 static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
  64 {
  65         return css_pids(pids->css.parent);
  66 }
  67
  68 static struct cgroup_subsys_state *
  69 pids_css_alloc(struct cgroup_subsys_state *parent)
  70 {
  71         struct pids_cgroup *pids;
  72
  73         pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
  74         if (!pids)
  75                 return ERR_PTR(-ENOMEM);
  76
  77         atomic64_set(&pids->counter, 0);
  78         atomic64_set(&pids->limit, PIDS_MAX);
  79         atomic64_set(&pids->events_limit, 0);
  80         return &pids->css;
  81 }
  82
  83 static void pids_css_free(struct cgroup_subsys_state *css)
  84 {
  85         kfree(css_pids(css));
  86 }
  87
  88 /**
  89  * pids_cancel - uncharge the local pid count
  90  * @pids: the pid cgroup state
  91  * @num: the number of pids to cancel
  92  *
  93  * This function will WARN if the pid count goes under 0, because such a case is
  94  * a bug in the pids controller proper.
  95  */
  96 static void pids_cancel(struct pids_cgroup *pids, int num)
  97 {
  98         /*
  99          * A negative count (or overflow for that matter) is invalid,
 100          * and indicates a bug in the `pids` controller proper.
 101          */
 102         WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
 103 }
 104
 105 /**
 106  * pids_uncharge - hierarchically uncharge the pid count
 107  * @pids: the pid cgroup state
 108  * @num: the number of pids to uncharge
 109  */
 110 static void pids_uncharge(struct pids_cgroup *pids, int num)
 111 {
 112         struct pids_cgroup *p;
 113
 114         for (p = pids; parent_pids(p); p = parent_pids(p))
 115                 pids_cancel(p, num);
 116 }
 117
 118 /**
 119  * pids_charge - hierarchically charge the pid count
 120  * @pids: the pid cgroup state
 121  * @num: the number of pids to charge
 122  *
 123  * This function does *not* follow the pid limit set. It cannot fail and the new
 124  * pid count may exceed the limit. This is only used for reverting failed
 125  * attaches, where there is no other way out than violating the limit.
 126  */
 127 static void pids_charge(struct pids_cgroup *pids, int num)
 128 {
 129         struct pids_cgroup *p;
 130
 131         for (p = pids; parent_pids(p); p = parent_pids(p))
 132                 atomic64_add(num, &p->counter);
 133 }
 134
 135 /**
 136  * pids_try_charge - hierarchically try to charge the pid count
 137  * @pids: the pid cgroup state
 138  * @num: the number of pids to charge
 139  *
 140  * This function follows the set limit. It will fail if the charge would cause
 141  * the new value to exceed the hierarchical limit. Returns 0 if the charge
 142  * succeeded, otherwise -EAGAIN.
 143  */
 144 static int pids_try_charge(struct pids_cgroup *pids, int num)
 145 {
 146         struct pids_cgroup *p, *q;
 147
 148         for (p = pids; parent_pids(p); p = parent_pids(p)) {
 149                 int64_t new = atomic64_add_return(num, &p->counter);
 150                 int64_t limit = atomic64_read(&p->limit);
 151
 152                 /*
 153                  * Since new is capped to the maximum number of pid_t, if
 154                  * p->limit is %PIDS_MAX then we know that this test will never
 155                  * fail.
 156                  */
 157                 if (new > limit)
 158                         goto revert;
 159         }
 160
 161         return 0;
 162
 163 revert:
 164         for (q = pids; q != p; q = parent_pids(q))
 165                 pids_cancel(q, num);
 166         pids_cancel(p, num);
 167
 168         return -EAGAIN;
 169 }
 170
 171 static int pids_can_attach(struct cgroup_taskset *tset)
 172 {
 173         struct task_struct *task;
 174         struct cgroup_subsys_state *dst_css;
 175
 176         cgroup_taskset_for_each(task, dst_css, tset) {
 177                 struct pids_cgroup *pids = css_pids(dst_css);
 178                 struct cgroup_subsys_state *old_css;
 179                 struct pids_cgroup *old_pids;
 180
 181                 /*
 182                  * No need to pin @old_css between here and cancel_attach()
 183                  * because cgroup core protects it from being freed before
 184                  * the migration completes or fails.
 185                  */
 186                 old_css = task_css(task, pids_cgrp_id);
 187                 old_pids = css_pids(old_css);
 188
 189                 pids_charge(pids, 1);
 190                 pids_uncharge(old_pids, 1);
 191         }
 192
 193         return 0;
 194 }
 195
 196 static void pids_cancel_attach(struct cgroup_taskset *tset)
 197 {
 198         struct task_struct *task;
 199         struct cgroup_subsys_state *dst_css;
 200
 201         cgroup_taskset_for_each(task, dst_css, tset) {
 202                 struct pids_cgroup *pids = css_pids(dst_css);
 203                 struct cgroup_subsys_state *old_css;
 204                 struct pids_cgroup *old_pids;
 205
 206                 old_css = task_css(task, pids_cgrp_id);
 207                 old_pids = css_pids(old_css);
 208
 209                 pids_charge(old_pids, 1);
 210                 pids_uncharge(pids, 1);
 211         }
 212 }
 213
 214 /*
 215  * task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
 216  * on cgroup_threadgroup_change_begin() held by the copy_process().
 217  */
 218 static int pids_can_fork(struct task_struct *task, struct css_set *cset)
 219 {
 220         struct cgroup_subsys_state *css;
 221         struct pids_cgroup *pids;
 222         int err;
 223
 224         if (cset)
 225                 css = cset->subsys[pids_cgrp_id];
 226         else
 227                 css = task_css_check(current, pids_cgrp_id, true);
 228         pids = css_pids(css);
 229         err = pids_try_charge(pids, 1);
 230         if (err) {
 231                 /* Only log the first time events_limit is incremented. */
 232                 if (atomic64_inc_return(&pids->events_limit) == 1) {
 233                         pr_info("cgroup: fork rejected by pids controller in ");
 234                         pr_cont_cgroup_path(css->cgroup);
 235                         pr_cont("\n");
 236                 }
 237                 cgroup_file_notify(&pids->events_file);
 238         }
 239         return err;
 240 }
 241
 242 static void pids_cancel_fork(struct task_struct *task, struct css_set *cset)
 243 {
 244         struct cgroup_subsys_state *css;
 245         struct pids_cgroup *pids;
 246
 247         if (cset)
 248                 css = cset->subsys[pids_cgrp_id];
 249         else
 250                 css = task_css_check(current, pids_cgrp_id, true);
 251         pids = css_pids(css);
 252         pids_uncharge(pids, 1);
 253 }
 254
 255 static void pids_release(struct task_struct *task)
 256 {
 257         struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
 258
 259         pids_uncharge(pids, 1);
 260 }
 261
 262 static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
 263                               size_t nbytes, loff_t off)
 264 {
 265         struct cgroup_subsys_state *css = of_css(of);
 266         struct pids_cgroup *pids = css_pids(css);
 267         int64_t limit;
 268         int err;
 269
 270         buf = strstrip(buf);
 271         if (!strcmp(buf, PIDS_MAX_STR)) {
 272                 limit = PIDS_MAX;
 273                 goto set_limit;
 274         }
 275
 276         err = kstrtoll(buf, 0, &limit);
 277         if (err)
 278                 return err;
 279
 280         if (limit < 0 || limit >= PIDS_MAX)
 281                 return -EINVAL;
 282
 283 set_limit:
 284         /*
 285          * Limit updates don't need to be mutex'd, since it isn't
 286          * critical that any racing fork()s follow the new limit.
 287          */
 288         atomic64_set(&pids->limit, limit);
 289         return nbytes;
 290 }
 291
 292 static int pids_max_show(struct seq_file *sf, void *v)
 293 {
 294         struct cgroup_subsys_state *css = seq_css(sf);
 295         struct pids_cgroup *pids = css_pids(css);
 296         int64_t limit = atomic64_read(&pids->limit);
 297
 298         if (limit >= PIDS_MAX)
 299                 seq_printf(sf, "%s\n", PIDS_MAX_STR);
 300         else
 301                 seq_printf(sf, "%lld\n", limit);
 302
 303         return 0;
 304 }
 305
 306 static s64 pids_current_read(struct cgroup_subsys_state *css,
 307                              struct cftype *cft)
 308 {
 309         struct pids_cgroup *pids = css_pids(css);
 310
 311         return atomic64_read(&pids->counter);
 312 }
 313
 314 static int pids_events_show(struct seq_file *sf, void *v)
 315 {
 316         struct pids_cgroup *pids = css_pids(seq_css(sf));
 317
 318         seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
 319         return 0;
 320 }
 321
 322 static struct cftype pids_files[] = {
 323         {
 324                 .name = "max",
 325                 .write = pids_max_write,
 326                 .seq_show = pids_max_show,
 327                 .flags = CFTYPE_NOT_ON_ROOT,
 328         },
 329         {
 330                 .name = "current",
 331                 .read_s64 = pids_current_read,
 332                 .flags = CFTYPE_NOT_ON_ROOT,
 333         },
 334         {
 335                 .name = "events",
 336                 .seq_show = pids_events_show,
 337                 .file_offset = offsetof(struct pids_cgroup, events_file),
 338                 .flags = CFTYPE_NOT_ON_ROOT,
 339         },
 340         { }     /* terminate */
 341 };
 342
 343 struct cgroup_subsys pids_cgrp_subsys = {
 344         .css_alloc      = pids_css_alloc,
 345         .css_free       = pids_css_free,
 346         .can_attach     = pids_can_attach,
 347         .cancel_attach  = pids_cancel_attach,
 348         .can_fork       = pids_can_fork,
 349         .cancel_fork    = pids_cancel_fork,
 350         .release        = pids_release,
 351         .legacy_cftypes = pids_files,
 352         .dfl_cftypes    = pids_files,
 353         .threaded       = true,
 354 };