349 lines
8.9 KiB
C
349 lines
8.9 KiB
C
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/*
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* Process number limiting controller for cgroups.
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*
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* Used to allow a cgroup hierarchy to stop any new processes from fork()ing
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* after a certain limit is reached.
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*
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* Since it is trivial to hit the task limit without hitting any kmemcg limits
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* in place, PIDs are a fundamental resource. As such, PID exhaustion must be
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* preventable in the scope of a cgroup hierarchy by allowing resource limiting
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* of the number of tasks in a cgroup.
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*
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* In order to use the `pids` controller, set the maximum number of tasks in
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* pids.max (this is not available in the root cgroup for obvious reasons). The
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* number of processes currently in the cgroup is given by pids.current.
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* Organisational operations are not blocked by cgroup policies, so it is
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* possible to have pids.current > pids.max. However, it is not possible to
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* violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
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* would cause a cgroup policy to be violated.
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*
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* To set a cgroup to have no limit, set pids.max to "max". This is the default
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* for all new cgroups (N.B. that PID limits are hierarchical, so the most
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* stringent limit in the hierarchy is followed).
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*
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* pids.current tracks all child cgroup hierarchies, so parent/pids.current is
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* a superset of parent/child/pids.current.
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*
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* Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
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*
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* This file is subject to the terms and conditions of version 2 of the GNU
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* General Public License. See the file COPYING in the main directory of the
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* Linux distribution for more details.
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*/
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#include <linux/kernel.h>
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#include <linux/threads.h>
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#include <linux/atomic.h>
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#include <linux/cgroup.h>
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#include <linux/slab.h>
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#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
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#define PIDS_MAX_STR "max"
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struct pids_cgroup {
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struct cgroup_subsys_state css;
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/*
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* Use 64-bit types so that we can safely represent "max" as
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* %PIDS_MAX = (%PID_MAX_LIMIT + 1).
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*/
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atomic64_t counter;
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int64_t limit;
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/* Handle for "pids.events" */
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struct cgroup_file events_file;
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/* Number of times fork failed because limit was hit. */
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atomic64_t events_limit;
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};
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static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
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{
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return container_of(css, struct pids_cgroup, css);
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}
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static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
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{
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return css_pids(pids->css.parent);
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}
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static struct cgroup_subsys_state *
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pids_css_alloc(struct cgroup_subsys_state *parent)
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{
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struct pids_cgroup *pids;
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pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
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if (!pids)
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return ERR_PTR(-ENOMEM);
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pids->limit = PIDS_MAX;
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atomic64_set(&pids->counter, 0);
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atomic64_set(&pids->events_limit, 0);
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return &pids->css;
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}
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static void pids_css_free(struct cgroup_subsys_state *css)
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{
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kfree(css_pids(css));
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}
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/**
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* pids_cancel - uncharge the local pid count
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* @pids: the pid cgroup state
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* @num: the number of pids to cancel
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*
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* This function will WARN if the pid count goes under 0, because such a case is
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* a bug in the pids controller proper.
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*/
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static void pids_cancel(struct pids_cgroup *pids, int num)
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{
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/*
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* A negative count (or overflow for that matter) is invalid,
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* and indicates a bug in the `pids` controller proper.
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*/
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WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
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}
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/**
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* pids_uncharge - hierarchically uncharge the pid count
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* @pids: the pid cgroup state
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* @num: the number of pids to uncharge
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*/
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static void pids_uncharge(struct pids_cgroup *pids, int num)
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{
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struct pids_cgroup *p;
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for (p = pids; parent_pids(p); p = parent_pids(p))
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pids_cancel(p, num);
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}
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/**
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* pids_charge - hierarchically charge the pid count
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* @pids: the pid cgroup state
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* @num: the number of pids to charge
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*
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* This function does *not* follow the pid limit set. It cannot fail and the new
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* pid count may exceed the limit. This is only used for reverting failed
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* attaches, where there is no other way out than violating the limit.
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*/
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static void pids_charge(struct pids_cgroup *pids, int num)
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{
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struct pids_cgroup *p;
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for (p = pids; parent_pids(p); p = parent_pids(p))
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atomic64_add(num, &p->counter);
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}
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/**
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* pids_try_charge - hierarchically try to charge the pid count
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* @pids: the pid cgroup state
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* @num: the number of pids to charge
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*
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* This function follows the set limit. It will fail if the charge would cause
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* the new value to exceed the hierarchical limit. Returns 0 if the charge
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* succeeded, otherwise -EAGAIN.
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*/
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static int pids_try_charge(struct pids_cgroup *pids, int num)
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{
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struct pids_cgroup *p, *q;
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for (p = pids; parent_pids(p); p = parent_pids(p)) {
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int64_t new = atomic64_add_return(num, &p->counter);
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/*
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* Since new is capped to the maximum number of pid_t, if
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* p->limit is %PIDS_MAX then we know that this test will never
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* fail.
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*/
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if (new > p->limit)
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goto revert;
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}
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return 0;
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revert:
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for (q = pids; q != p; q = parent_pids(q))
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pids_cancel(q, num);
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pids_cancel(p, num);
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return -EAGAIN;
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}
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static int pids_can_attach(struct cgroup_taskset *tset)
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{
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struct task_struct *task;
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struct cgroup_subsys_state *dst_css;
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cgroup_taskset_for_each(task, dst_css, tset) {
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struct pids_cgroup *pids = css_pids(dst_css);
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struct cgroup_subsys_state *old_css;
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struct pids_cgroup *old_pids;
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/*
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* No need to pin @old_css between here and cancel_attach()
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* because cgroup core protects it from being freed before
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* the migration completes or fails.
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*/
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old_css = task_css(task, pids_cgrp_id);
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old_pids = css_pids(old_css);
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pids_charge(pids, 1);
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pids_uncharge(old_pids, 1);
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}
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return 0;
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}
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static void pids_cancel_attach(struct cgroup_taskset *tset)
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{
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struct task_struct *task;
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struct cgroup_subsys_state *dst_css;
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cgroup_taskset_for_each(task, dst_css, tset) {
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struct pids_cgroup *pids = css_pids(dst_css);
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struct cgroup_subsys_state *old_css;
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struct pids_cgroup *old_pids;
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old_css = task_css(task, pids_cgrp_id);
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old_pids = css_pids(old_css);
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pids_charge(old_pids, 1);
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pids_uncharge(pids, 1);
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}
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}
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/*
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* task_css_check(true) in pids_can_fork() and pids_cancel_fork() relies
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* on threadgroup_change_begin() held by the copy_process().
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*/
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static int pids_can_fork(struct task_struct *task)
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{
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struct cgroup_subsys_state *css;
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struct pids_cgroup *pids;
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int err;
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css = task_css_check(current, pids_cgrp_id, true);
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pids = css_pids(css);
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err = pids_try_charge(pids, 1);
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if (err) {
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/* Only log the first time events_limit is incremented. */
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if (atomic64_inc_return(&pids->events_limit) == 1) {
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pr_info("cgroup: fork rejected by pids controller in ");
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pr_cont_cgroup_path(css->cgroup);
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pr_cont("\n");
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}
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cgroup_file_notify(&pids->events_file);
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}
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return err;
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}
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static void pids_cancel_fork(struct task_struct *task)
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{
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struct cgroup_subsys_state *css;
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struct pids_cgroup *pids;
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css = task_css_check(current, pids_cgrp_id, true);
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pids = css_pids(css);
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pids_uncharge(pids, 1);
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}
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static void pids_free(struct task_struct *task)
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{
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struct pids_cgroup *pids = css_pids(task_css(task, pids_cgrp_id));
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pids_uncharge(pids, 1);
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}
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static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
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size_t nbytes, loff_t off)
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{
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struct cgroup_subsys_state *css = of_css(of);
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struct pids_cgroup *pids = css_pids(css);
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int64_t limit;
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int err;
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buf = strstrip(buf);
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if (!strcmp(buf, PIDS_MAX_STR)) {
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limit = PIDS_MAX;
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goto set_limit;
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}
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err = kstrtoll(buf, 0, &limit);
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if (err)
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return err;
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if (limit < 0 || limit >= PIDS_MAX)
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return -EINVAL;
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set_limit:
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/*
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* Limit updates don't need to be mutex'd, since it isn't
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* critical that any racing fork()s follow the new limit.
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*/
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pids->limit = limit;
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return nbytes;
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}
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static int pids_max_show(struct seq_file *sf, void *v)
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{
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struct cgroup_subsys_state *css = seq_css(sf);
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struct pids_cgroup *pids = css_pids(css);
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int64_t limit = pids->limit;
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if (limit >= PIDS_MAX)
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seq_printf(sf, "%s\n", PIDS_MAX_STR);
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else
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seq_printf(sf, "%lld\n", limit);
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return 0;
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}
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static s64 pids_current_read(struct cgroup_subsys_state *css,
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struct cftype *cft)
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{
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struct pids_cgroup *pids = css_pids(css);
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return atomic64_read(&pids->counter);
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}
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static int pids_events_show(struct seq_file *sf, void *v)
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{
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struct pids_cgroup *pids = css_pids(seq_css(sf));
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seq_printf(sf, "max %lld\n", (s64)atomic64_read(&pids->events_limit));
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return 0;
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}
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static struct cftype pids_files[] = {
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{
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.name = "max",
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.write = pids_max_write,
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.seq_show = pids_max_show,
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.flags = CFTYPE_NOT_ON_ROOT,
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},
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{
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.name = "current",
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.read_s64 = pids_current_read,
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.flags = CFTYPE_NOT_ON_ROOT,
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},
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{
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.name = "events",
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.seq_show = pids_events_show,
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.file_offset = offsetof(struct pids_cgroup, events_file),
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.flags = CFTYPE_NOT_ON_ROOT,
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},
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{ } /* terminate */
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};
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struct cgroup_subsys pids_cgrp_subsys = {
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.css_alloc = pids_css_alloc,
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.css_free = pids_css_free,
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.can_attach = pids_can_attach,
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.cancel_attach = pids_cancel_attach,
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.can_fork = pids_can_fork,
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.cancel_fork = pids_cancel_fork,
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.free = pids_free,
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.legacy_cftypes = pids_files,
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.dfl_cftypes = pids_files,
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};
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