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Subject[PATCH v14 3/4] cgroup: allow a cgroup subsystem to reject a fork
Date
Add a new cgroup subsystem callback can_fork that conditionally
states whether or not the fork is accepted or rejected by a cgroup
policy. In addition, add a cancel_fork callback so that if an error
occurs later in the forking process, any state modified by can_fork can
be reverted.

Allow for a private opaque pointer to be passed from cgroup_can_fork to
cgroup_post_fork, allowing for the fork state to be stored by each
subsystem separately.

Also add a tagging system for cgroup_subsys.h to allow for CGROUP_<TAG>
enumerations to be be defined and used. In addition, explicitly add a
CGROUP_CANFORK_COUNT macro to make arrays easier to define.

This is in preparation for implementing the pids cgroup subsystem.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
---
include/linux/cgroup-defs.h | 10 +++++-
include/linux/cgroup.h | 15 +++++++--
include/linux/cgroup_subsys.h | 23 ++++++++++++++
kernel/cgroup.c | 73 +++++++++++++++++++++++++++++++++++++++++--
kernel/cgroup_freezer.c | 2 +-
kernel/fork.c | 17 ++++++++--
kernel/sched/core.c | 2 +-
7 files changed, 133 insertions(+), 9 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index c5588c4..2009ceb 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -34,12 +34,17 @@ struct seq_file;

/* define the enumeration of all cgroup subsystems */
#define SUBSYS(_x) _x ## _cgrp_id,
+#define SUBSYS_TAG(_t) CGROUP_ ## _t, \
+ __unused_tag_ ## _t = CGROUP_ ## _t - 1,
enum cgroup_subsys_id {
#include <linux/cgroup_subsys.h>
CGROUP_SUBSYS_COUNT,
};
+#undef SUBSYS_TAG
#undef SUBSYS

+#define CGROUP_CANFORK_COUNT (CGROUP_CANFORK_END - CGROUP_CANFORK_START)
+
/* bits in struct cgroup_subsys_state flags field */
enum {
CSS_NO_REF = (1 << 0), /* no reference counting for this css */
@@ -405,7 +410,9 @@ struct cgroup_subsys {
struct cgroup_taskset *tset);
void (*attach)(struct cgroup_subsys_state *css,
struct cgroup_taskset *tset);
- void (*fork)(struct task_struct *task);
+ int (*can_fork)(struct task_struct *task, void **priv_p);
+ void (*cancel_fork)(struct task_struct *task, void *priv);
+ void (*fork)(struct task_struct *task, void *priv);
void (*exit)(struct cgroup_subsys_state *css,
struct cgroup_subsys_state *old_css,
struct task_struct *task);
@@ -490,6 +497,7 @@ static inline void cgroup_threadgroup_change_end(struct task_struct *tsk)

#else /* CONFIG_CGROUPS */

+#define CGROUP_CANFORK_COUNT 0
#define CGROUP_SUBSYS_COUNT 0

static inline void cgroup_threadgroup_change_begin(struct task_struct *tsk) {}
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index a593e29..a71fe2a 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -62,7 +62,12 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
struct pid *pid, struct task_struct *tsk);

void cgroup_fork(struct task_struct *p);
-void cgroup_post_fork(struct task_struct *p);
+extern int cgroup_can_fork(struct task_struct *p,
+ void *ss_priv[CGROUP_CANFORK_COUNT]);
+extern void cgroup_cancel_fork(struct task_struct *p,
+ void *ss_priv[CGROUP_CANFORK_COUNT]);
+extern void cgroup_post_fork(struct task_struct *p,
+ void *old_ss_priv[CGROUP_CANFORK_COUNT]);
void cgroup_exit(struct task_struct *p);

int cgroup_init_early(void);
@@ -524,7 +529,13 @@ static inline int cgroupstats_build(struct cgroupstats *stats,
struct dentry *dentry) { return -EINVAL; }

static inline void cgroup_fork(struct task_struct *p) {}
-static inline void cgroup_post_fork(struct task_struct *p) {}
+static inline int cgroup_can_fork(struct task_struct *p,
+ void *ss_priv[CGROUP_CANFORK_COUNT])
+{ return 0; }
+static inline void cgroup_cancel_fork(struct task_struct *p,
+ void *ss_priv[CGROUP_CANFORK_COUNT]) {}
+static inline void cgroup_post_fork(struct task_struct *p,
+ void *ss_priv[CGROUP_CANFORK_COUNT]) {}
static inline void cgroup_exit(struct task_struct *p) {}

static inline int cgroup_init_early(void) { return 0; }
diff --git a/include/linux/cgroup_subsys.h b/include/linux/cgroup_subsys.h
index e4a96fb..ec43bce 100644
--- a/include/linux/cgroup_subsys.h
+++ b/include/linux/cgroup_subsys.h
@@ -3,6 +3,17 @@
*
* DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS.
*/
+
+/*
+ * This file *must* be included with SUBSYS() defined.
+ * SUBSYS_TAG() is a noop if undefined.
+ */
+
+#ifndef SUBSYS_TAG
+#define __TMP_SUBSYS_TAG
+#define SUBSYS_TAG(_x)
+#endif
+
#if IS_ENABLED(CONFIG_CPUSETS)
SUBSYS(cpuset)
#endif
@@ -48,11 +59,23 @@ SUBSYS(hugetlb)
#endif

/*
+ * Subsystems that implement the can_fork() family of callbacks.
+ */
+SUBSYS_TAG(CANFORK_START)
+SUBSYS_TAG(CANFORK_END)
+
+/*
* The following subsystems are not supported on the default hierarchy.
*/
#if IS_ENABLED(CONFIG_CGROUP_DEBUG)
SUBSYS(debug)
#endif
+
+#ifdef __TMP_SUBSYS_TAG
+#undef __TMP_SUBSYS_TAG
+#undef SUBSYS_TAG
+#endif
+
/*
* DO NOT ADD ANY SUBSYSTEM WITHOUT EXPLICIT ACKS FROM CGROUP MAINTAINERS.
*/
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 12b580f..ee63ea2 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -186,6 +186,9 @@ static u64 css_serial_nr_next = 1;
static unsigned long have_fork_callback __read_mostly;
static unsigned long have_exit_callback __read_mostly;

+/* Ditto for the can_fork callback. */
+static unsigned long have_canfork_callback __read_mostly;
+
static struct cftype cgroup_dfl_base_files[];
static struct cftype cgroup_legacy_base_files[];

@@ -4923,6 +4926,7 @@ static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)

have_fork_callback |= (bool)ss->fork << ss->id;
have_exit_callback |= (bool)ss->exit << ss->id;
+ have_canfork_callback |= (bool)ss->can_fork << ss->id;

/* At system boot, before all subsystems have been
* registered, no tasks have been forked, so we don't
@@ -5165,6 +5169,19 @@ static const struct file_operations proc_cgroupstats_operations = {
.release = single_release,
};

+static void **subsys_canfork_priv_p(void *ss_priv[CGROUP_CANFORK_COUNT], int i)
+{
+ if (CGROUP_CANFORK_START <= i && i < CGROUP_CANFORK_END)
+ return &ss_priv[i - CGROUP_CANFORK_START];
+ return NULL;
+}
+
+static void *subsys_canfork_priv(void *ss_priv[CGROUP_CANFORK_COUNT], int i)
+{
+ void **private = subsys_canfork_priv_p(ss_priv, i);
+ return private ? *private : NULL;
+}
+
/**
* cgroup_fork - initialize cgroup related fields during copy_process()
* @child: pointer to task_struct of forking parent process.
@@ -5180,6 +5197,57 @@ void cgroup_fork(struct task_struct *child)
}

/**
+ * cgroup_can_fork - called on a new task before the process is exposed
+ * @child: the task in question.
+ *
+ * This calls the subsystem can_fork() callbacks. If the can_fork() callback
+ * returns an error, the fork aborts with that error code. This allows for
+ * a cgroup subsystem to conditionally allow or deny new forks.
+ */
+int cgroup_can_fork(struct task_struct *child,
+ void *ss_priv[CGROUP_CANFORK_COUNT])
+{
+ struct cgroup_subsys *ss;
+ int i, j, ret;
+
+ for_each_subsys_which(ss, i, &have_canfork_callback) {
+ ret = ss->can_fork(child, subsys_canfork_priv_p(ss_priv, i));
+ if (ret)
+ goto out_revert;
+ }
+
+ return 0;
+
+out_revert:
+ for_each_subsys(ss, j) {
+ if (j >= i)
+ break;
+ if (ss->cancel_fork)
+ ss->cancel_fork(child, subsys_canfork_priv(ss_priv, j));
+ }
+
+ return ret;
+}
+
+/**
+ * cgroup_cancel_fork - called if a fork failed after cgroup_can_fork()
+ * @child: the task in question
+ *
+ * This calls the cancel_fork() callbacks if a fork failed *after*
+ * cgroup_can_fork() succeded.
+ */
+void cgroup_cancel_fork(struct task_struct *child,
+ void *ss_priv[CGROUP_CANFORK_COUNT])
+{
+ struct cgroup_subsys *ss;
+ int i;
+
+ for_each_subsys(ss, i)
+ if (ss->cancel_fork)
+ ss->cancel_fork(child, subsys_canfork_priv(ss_priv, i));
+}
+
+/**
* cgroup_post_fork - called on a new task after adding it to the task list
* @child: the task in question
*
@@ -5189,7 +5257,8 @@ void cgroup_fork(struct task_struct *child)
* cgroup_task_iter_start() - to guarantee that the new task ends up on its
* list.
*/
-void cgroup_post_fork(struct task_struct *child)
+void cgroup_post_fork(struct task_struct *child,
+ void *old_ss_priv[CGROUP_CANFORK_COUNT])
{
struct cgroup_subsys *ss;
int i;
@@ -5234,7 +5303,7 @@ void cgroup_post_fork(struct task_struct *child)
* and addition to css_set.
*/
for_each_subsys_which(ss, i, &have_fork_callback)
- ss->fork(child);
+ ss->fork(child, subsys_canfork_priv(old_ss_priv, i));
}

/**
diff --git a/kernel/cgroup_freezer.c b/kernel/cgroup_freezer.c
index 92b98cc..f1b30ad 100644
--- a/kernel/cgroup_freezer.c
+++ b/kernel/cgroup_freezer.c
@@ -203,7 +203,7 @@ static void freezer_attach(struct cgroup_subsys_state *new_css,
* to do anything as freezer_attach() will put @task into the appropriate
* state.
*/
-static void freezer_fork(struct task_struct *task)
+static void freezer_fork(struct task_struct *task, void *private)
{
struct freezer *freezer;

diff --git a/kernel/fork.c b/kernel/fork.c
index 9531275..de7ff51 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1241,6 +1241,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
{
int retval;
struct task_struct *p;
+ void *cgrp_ss_priv[CGROUP_CANFORK_COUNT] = {};

if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
return ERR_PTR(-EINVAL);
@@ -1512,6 +1513,16 @@ static struct task_struct *copy_process(unsigned long clone_flags,
p->task_works = NULL;

/*
+ * Ensure that the cgroup subsystem policies allow the new process to be
+ * forked. It should be noted the the new process's css_set can be changed
+ * between here and cgroup_post_fork() if an organisation operation is in
+ * progress.
+ */
+ retval = cgroup_can_fork(p, cgrp_ss_priv);
+ if (retval)
+ goto bad_fork_free_pid;
+
+ /*
* Make it visible to the rest of the system, but dont wake it up yet.
* Need tasklist lock for parent etc handling!
*/
@@ -1547,7 +1558,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
spin_unlock(&current->sighand->siglock);
write_unlock_irq(&tasklist_lock);
retval = -ERESTARTNOINTR;
- goto bad_fork_free_pid;
+ goto bad_fork_cancel_cgroup;
}

if (likely(p->pid)) {
@@ -1589,7 +1600,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
write_unlock_irq(&tasklist_lock);

proc_fork_connector(p);
- cgroup_post_fork(p);
+ cgroup_post_fork(p, cgrp_ss_priv);
if (clone_flags & CLONE_THREAD)
threadgroup_change_end(current);
perf_event_fork(p);
@@ -1599,6 +1610,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,

return p;

+bad_fork_cancel_cgroup:
+ cgroup_cancel_fork(p, cgrp_ss_priv);
bad_fork_free_pid:
if (pid != &init_struct_pid)
free_pid(pid);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f9123a8..050936e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8007,7 +8007,7 @@ static void cpu_cgroup_css_offline(struct cgroup_subsys_state *css)
sched_offline_group(tg);
}

-static void cpu_cgroup_fork(struct task_struct *task)
+static void cpu_cgroup_fork(struct task_struct *task, void *private)
{
sched_move_task(task);
}
--
2.4.2


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