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-rw-r--r--CREDITS5
-rw-r--r--include/linux/cgroup_subsys.h5
-rw-r--r--init/Kconfig16
-rw-r--r--kernel/Makefile1
-rw-r--r--kernel/cgroup_pids.c366
5 files changed, 393 insertions, 0 deletions
diff --git a/CREDITS b/CREDITS
index 1d61664..4fcf9cd 100644
--- a/CREDITS
+++ b/CREDITS
@@ -3219,6 +3219,11 @@ S: 69 rue Dunois
S: 75013 Paris
S: France
+N: Aleksa Sarai
+E: cyphar@cyphar.com
+W: https://www.cyphar.com/
+D: `pids` cgroup subsystem
+
N: Dipankar Sarma
E: dipankar@in.ibm.com
D: RCU
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index ec43bce..1f36945 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -62,6 +62,11 @@ SUBSYS(hugetlb)
* Subsystems that implement the can_fork() family of callbacks.
*/
SUBSYS_TAG(CANFORK_START)
+
+#if IS_ENABLED(CONFIG_CGROUP_PIDS)
+SUBSYS(pids)
+#endif
+
SUBSYS_TAG(CANFORK_END)
/*
diff --git a/init/Kconfig b/init/Kconfig
index af09b4f..2184b34 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -955,6 +955,22 @@ config CGROUP_FREEZER
Provides a way to freeze and unfreeze all tasks in a
cgroup.
+config CGROUP_PIDS
+ bool "PIDs cgroup subsystem"
+ help
+ Provides enforcement of process number limits in the scope of a
+ cgroup. Any attempt to fork more processes than is allowed in the
+ cgroup will fail. PIDs are fundamentally a global resource because it
+ is fairly trivial to reach PID exhaustion before you reach even a
+ conservative kmemcg limit. As a result, it is possible to grind a
+ system to halt without being limited by other cgroup policies. The
+ PIDs cgroup subsystem is designed to stop this from happening.
+
+ It should be noted that organisational operations (such as attaching
+ to a cgroup hierarchy will *not* be blocked by the PIDs subsystem),
+ since the PIDs limit only affects a process's ability to fork, not to
+ attach to a cgroup.
+
config CGROUP_DEVICE
bool "Device controller for cgroups"
help
diff --git a/kernel/Makefile b/kernel/Makefile
index 43c4c92..718fb8a 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -55,6 +55,7 @@ obj-$(CONFIG_BACKTRACE_SELF_TEST) += backtracetest.o
obj-$(CONFIG_COMPAT) += compat.o
obj-$(CONFIG_CGROUPS) += cgroup.o
obj-$(CONFIG_CGROUP_FREEZER) += cgroup_freezer.o
+obj-$(CONFIG_CGROUP_PIDS) += cgroup_pids.o
obj-$(CONFIG_CPUSETS) += cpuset.o
obj-$(CONFIG_UTS_NS) += utsname.o
obj-$(CONFIG_USER_NS) += user_namespace.o
diff --git a/kernel/cgroup_pids.c b/kernel/cgroup_pids.c
new file mode 100644
index 0000000..d754888
--- /dev/null
+++ b/kernel/cgroup_pids.c
@@ -0,0 +1,366 @@
+/*
+ * Process number limiting controller for cgroups.
+ *
+ * Used to allow a cgroup hierarchy to stop any new processes from fork()ing
+ * after a certain limit is reached.
+ *
+ * Since it is trivial to hit the task limit without hitting any kmemcg limits
+ * in place, PIDs are a fundamental resource. As such, PID exhaustion must be
+ * preventable in the scope of a cgroup hierarchy by allowing resource limiting
+ * of the number of tasks in a cgroup.
+ *
+ * In order to use the `pids` controller, set the maximum number of tasks in
+ * pids.max (this is not available in the root cgroup for obvious reasons). The
+ * number of processes currently in the cgroup is given by pids.current.
+ * Organisational operations are not blocked by cgroup policies, so it is
+ * possible to have pids.current > pids.max. However, it is not possible to
+ * violate a cgroup policy through fork(). fork() will return -EAGAIN if forking
+ * would cause a cgroup policy to be violated.
+ *
+ * To set a cgroup to have no limit, set pids.max to "max". This is the default
+ * for all new cgroups (N.B. that PID limits are hierarchical, so the most
+ * stringent limit in the hierarchy is followed).
+ *
+ * pids.current tracks all child cgroup hierarchies, so parent/pids.current is
+ * a superset of parent/child/pids.current.
+ *
+ * Copyright (C) 2015 Aleksa Sarai <cyphar@cyphar.com>
+ *
+ * This file is subject to the terms and conditions of version 2 of the GNU
+ * General Public License. See the file COPYING in the main directory of the
+ * Linux distribution for more details.
+ */
+
+#include <linux/kernel.h>
+#include <linux/threads.h>
+#include <linux/atomic.h>
+#include <linux/cgroup.h>
+#include <linux/slab.h>
+
+#define PIDS_MAX (PID_MAX_LIMIT + 1ULL)
+#define PIDS_MAX_STR "max"
+
+struct pids_cgroup {
+ struct cgroup_subsys_state css;
+
+ /*
+ * Use 64-bit types so that we can safely represent "max" as
+ * %PIDS_MAX = (%PID_MAX_LIMIT + 1).
+ */
+ atomic64_t counter;
+ int64_t limit;
+};
+
+static struct pids_cgroup *css_pids(struct cgroup_subsys_state *css)
+{
+ return container_of(css, struct pids_cgroup, css);
+}
+
+static struct pids_cgroup *parent_pids(struct pids_cgroup *pids)
+{
+ return css_pids(pids->css.parent);
+}
+
+static struct cgroup_subsys_state *
+pids_css_alloc(struct cgroup_subsys_state *parent)
+{
+ struct pids_cgroup *pids;
+
+ pids = kzalloc(sizeof(struct pids_cgroup), GFP_KERNEL);
+ if (!pids)
+ return ERR_PTR(-ENOMEM);
+
+ pids->limit = PIDS_MAX;
+ atomic64_set(&pids->counter, 0);
+ return &pids->css;
+}
+
+static void pids_css_free(struct cgroup_subsys_state *css)
+{
+ kfree(css_pids(css));
+}
+
+/**
+ * pids_cancel - uncharge the local pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to cancel
+ *
+ * This function will WARN if the pid count goes under 0, because such a case is
+ * a bug in the pids controller proper.
+ */
+static void pids_cancel(struct pids_cgroup *pids, int num)
+{
+ /*
+ * A negative count (or overflow for that matter) is invalid,
+ * and indicates a bug in the `pids` controller proper.
+ */
+ WARN_ON_ONCE(atomic64_add_negative(-num, &pids->counter));
+}
+
+/**
+ * pids_uncharge - hierarchically uncharge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to uncharge
+ */
+static void pids_uncharge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p;
+
+ for (p = pids; p; p = parent_pids(p))
+ pids_cancel(p, num);
+}
+
+/**
+ * pids_charge - hierarchically charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function does *not* follow the pid limit set. It cannot fail and the new
+ * pid count may exceed the limit. This is only used for reverting failed
+ * attaches, where there is no other way out than violating the limit.
+ */
+static void pids_charge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p;
+
+ for (p = pids; p; p = parent_pids(p))
+ atomic64_add(num, &p->counter);
+}
+
+/**
+ * pids_try_charge - hierarchically try to charge the pid count
+ * @pids: the pid cgroup state
+ * @num: the number of pids to charge
+ *
+ * This function follows the set limit. It will fail if the charge would cause
+ * the new value to exceed the hierarchical limit. Returns 0 if the charge
+ * succeded, otherwise -EAGAIN.
+ */
+static int pids_try_charge(struct pids_cgroup *pids, int num)
+{
+ struct pids_cgroup *p, *q;
+
+ for (p = pids; p; p = parent_pids(p)) {
+ int64_t new = atomic64_add_return(num, &p->counter);
+
+ /*
+ * Since new is capped to the maximum number of pid_t, if
+ * p->limit is %PIDS_MAX then we know that this test will never
+ * fail.
+ */
+ if (new > p->limit)
+ goto revert;
+ }
+
+ return 0;
+
+revert:
+ for (q = pids; q != p; q = parent_pids(q))
+ pids_cancel(q, num);
+ pids_cancel(p, num);
+
+ return -EAGAIN;
+}
+
+static int pids_can_attach(struct cgroup_subsys_state *css,
+ struct cgroup_taskset *tset)
+{
+ struct pids_cgroup *pids = css_pids(css);
+ struct task_struct *task;
+
+ cgroup_taskset_for_each(task, tset) {
+ struct cgroup_subsys_state *old_css;
+ struct pids_cgroup *old_pids;
+
+ /*
+ * Grab a ref to each task's css. We don't drop the ref until
+ * we either fail and hit ->cancel_attach() or succeed and hit
+ * ->attach().
+ */
+ old_css = task_get_css(task, pids_cgrp_id);
+ old_pids = css_pids(old_css);
+
+ pids_charge(pids, 1);
+ pids_uncharge(old_pids, 1);
+ }
+
+ return 0;
+}
+
+static void pids_cancel_attach(struct cgroup_subsys_state *css,
+ struct cgroup_taskset *tset)
+{
+ struct pids_cgroup *pids = css_pids(css);
+ struct task_struct *task;
+
+ cgroup_taskset_for_each(task, tset) {
+ struct cgroup_subsys_state *old_css;
+ struct pids_cgroup *old_pids;
+
+ old_css = task_css(task, pids_cgrp_id);
+ old_pids = css_pids(old_css);
+
+ pids_charge(old_pids, 1);
+ pids_uncharge(pids, 1);
+ css_put(old_css);
+ }
+}
+
+static void pids_attach(struct cgroup_subsys_state *css,
+ struct cgroup_taskset *tset)
+{
+ struct task_struct *task;
+
+ cgroup_taskset_for_each(task, tset)
+ css_put(task_css(task, pids_cgrp_id));
+}
+
+static int pids_can_fork(struct task_struct *task, void **priv_p)
+{
+ struct cgroup_subsys_state *css;
+ struct pids_cgroup *pids;
+ int err;
+
+ /*
+ * Use the "current" task_css for the pids subsystem as the tentative
+ * css. It is possible we will charge the wrong hierarchy, in which
+ * case we will forcefully revert/reapply the charge on the right
+ * hierarchy after it is committed to the task proper.
+ */
+ css = task_get_css(current, pids_cgrp_id);
+ pids = css_pids(css);
+
+ err = pids_try_charge(pids, 1);
+ if (err)
+ goto err_css_put;
+
+ *priv_p = css;
+ return 0;
+
+err_css_put:
+ css_put(css);
+ return err;
+}
+
+static void pids_cancel_fork(struct task_struct *task, void *priv)
+{
+ struct cgroup_subsys_state *css = priv;
+ struct pids_cgroup *pids = css_pids(css);
+
+ pids_uncharge(pids, 1);
+ css_put(css);
+}
+
+static void pids_fork(struct task_struct *task, void *priv)
+{
+ struct cgroup_subsys_state *css;
+ struct cgroup_subsys_state *old_css = priv;
+ struct pids_cgroup *pids;
+ struct pids_cgroup *old_pids = css_pids(old_css);
+
+ css = task_get_css(task, pids_cgrp_id);
+ pids = css_pids(css);
+
+ /*
+ * If the association has changed, we have to revert and reapply the
+ * charge/uncharge on the wrong hierarchy to the current one. Since
+ * the association can only change due to an organisation event, its
+ * okay for us to ignore the limit in this case.
+ */
+ if (pids != old_pids) {
+ pids_uncharge(old_pids, 1);
+ pids_charge(pids, 1);
+ }
+
+ css_put(css);
+ css_put(old_css);
+}
+
+static void pids_exit(struct cgroup_subsys_state *css,
+ struct cgroup_subsys_state *old_css,
+ struct task_struct *task)
+{
+ struct pids_cgroup *pids = css_pids(old_css);
+
+ pids_uncharge(pids, 1);
+}
+
+static ssize_t pids_max_write(struct kernfs_open_file *of, char *buf,
+ size_t nbytes, loff_t off)
+{
+ struct cgroup_subsys_state *css = of_css(of);
+ struct pids_cgroup *pids = css_pids(css);
+ int64_t limit;
+ int err;
+
+ buf = strstrip(buf);
+ if (!strcmp(buf, PIDS_MAX_STR)) {
+ limit = PIDS_MAX;
+ goto set_limit;
+ }
+
+ err = kstrtoll(buf, 0, &limit);
+ if (err)
+ return err;
+
+ if (limit < 0 || limit >= PIDS_MAX)
+ return -EINVAL;
+
+set_limit:
+ /*
+ * Limit updates don't need to be mutex'd, since it isn't
+ * critical that any racing fork()s follow the new limit.
+ */
+ pids->limit = limit;
+ return nbytes;
+}
+
+static int pids_max_show(struct seq_file *sf, void *v)
+{
+ struct cgroup_subsys_state *css = seq_css(sf);
+ struct pids_cgroup *pids = css_pids(css);
+ int64_t limit = pids->limit;
+
+ if (limit >= PIDS_MAX)
+ seq_printf(sf, "%s\n", PIDS_MAX_STR);
+ else
+ seq_printf(sf, "%lld\n", limit);
+
+ return 0;
+}
+
+static s64 pids_current_read(struct cgroup_subsys_state *css,
+ struct cftype *cft)
+{
+ struct pids_cgroup *pids = css_pids(css);
+
+ return atomic64_read(&pids->counter);
+}
+
+static struct cftype pids_files[] = {
+ {
+ .name = "max",
+ .write = pids_max_write,
+ .seq_show = pids_max_show,
+ .flags = CFTYPE_NOT_ON_ROOT,
+ },
+ {
+ .name = "current",
+ .read_s64 = pids_current_read,
+ },
+ { } /* terminate */
+};
+
+struct cgroup_subsys pids_cgrp_subsys = {
+ .css_alloc = pids_css_alloc,
+ .css_free = pids_css_free,
+ .attach = pids_attach,
+ .can_attach = pids_can_attach,
+ .cancel_attach = pids_cancel_attach,
+ .can_fork = pids_can_fork,
+ .cancel_fork = pids_cancel_fork,
+ .fork = pids_fork,
+ .exit = pids_exit,
+ .legacy_cftypes = pids_files,
+ .dfl_cftypes = pids_files,
+};
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