linux/kernel/cgroup/pids.c
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   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};
 347