Re: [PATCH 1/5] cgroup: introduce cgroup.kill

[Christian Brauner <christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>]

On Thu, Apr 29, 2021 at 08:20:11PM -0700, Roman Gushchin wrote:
> On Thu, Apr 29, 2021 at 02:01:09PM +0200, Christian Brauner wrote:
> > From: Christian Brauner <christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
> > 
> > Introduce the cgroup.kill file. It does what it says on the tin and
> > allows a caller to kill a cgroup by writing "1" into cgroup.kill.
> > The file is available in non-root cgroups.
> > 
> > Killing cgroups is a process directed operation, i.e. the whole
> > thread-group is affected. Consequently trying to write to cgroup.kill in
> > threaded cgroups will be rejected and EOPNOTSUPP returned. This behavior
> > aligns with cgroup.procs where reads in threaded-cgroups are rejected
> > with EOPNOTSUPP.
> > 
> > The cgroup.kill file is write-only since killing a cgroup is an event
> > not which makes it different from e.g. freezer where a cgroup
> > transitions between the two states.
> > 
> > As with all new cgroup features cgroup.kill is recursive by default.
> > 
> > Killing a cgroup is protected against concurrent migrations through the
> > cgroup mutex. To protect against forkbombs and to mitigate the effect of
> > racing forks a new CGRP_KILL css set lock protected flag is introduced
> > that is set prior to killing a cgroup and unset after the cgroup has
> > been killed. We can then check in cgroup_post_fork() where we hold the
> > css set lock already whether the cgroup is currently being killed. If so
> > we send the child a SIGKILL signal immediately taking it down as soon as
> > it returns to userspace. To make the killing of the child semantically
> > clean it is killed after all cgroup attachment operations have been
> > finalized.
> > 
> > There are various use-cases of this interface:
> > - Containers usually have a conservative layout where each container
> >   usually has a delegated cgroup. For such layouts there is a 1:1
> >   mapping between container and cgroup. If the container in addition
> >   uses a separate pid namespace then killing a container usually becomes
> >   a simple kill -9 <container-init-pid> from an ancestor pid namespace.
> >   However, there are quite a few scenarios where that isn't true. For
> >   example, there are containers that share the cgroup with other
> >   processes on purpose that are supposed to be bound to the lifetime of
> >   the container but are not in the same pidns of the container.
> >   Containers that are in a delegated cgroup but share the pid namespace
> >   with the host or other containers.
> > - Service managers such as systemd use cgroups to group and organize
> >   processes belonging to a service. They usually rely on a recursive
> >   algorithm now to kill a service. With cgroup.kill this becomes a
> >   simple write to cgroup.kill.
> > - Userspace OOM implementations can make good use of this feature to
> >   efficiently take down whole cgroups quickly.
> > - The kill program can gain a new
> >   kill --cgroup /sys/fs/cgroup/delegated
> >   flag to take down cgroups.
> > 
> > A few observations about the semantics:
> > - If parent and child are in the same cgroup and CLONE_INTO_CGROUP is
> >   not specified we are not taking cgroup mutex meaning the cgroup can be
> >   killed while a process in that cgroup is forking.
> >   If the kill request happens right before cgroup_can_fork() and before
> >   the parent grabs its siglock the parent is guaranteed to see the
> >   pending SIGKILL. In addition we perform another check in
> >   cgroup_post_fork() whether the cgroup is being killed and is so take
> >   down the child (see above). This is robust enough and protects gainst
> >   forkbombs. If userspace really really wants to have stricter
> >   protection the simple solution would be to grab the write side of the
> >   cgroup threadgroup rwsem which will force all ongoing forks to
> >   complete before killing starts. We concluded that this is not
> >   necessary as the semantics for concurrent forking should simply align
> >   with freezer where a similar check as cgroup_post_fork() is performed.
> > 
> >   For all other cases CLONE_INTO_CGROUP is required. In this case we
> >   will grab the cgroup mutex so the cgroup can't be killed while we
> >   fork. Once we're done with the fork and have dropped cgroup mutex we
> >   are visible and will be found by any subsequent kill request.
> > 
> > - We obviously don't kill kthreads. This means a cgroup that has a
> >   kthread will not become empty after killing and consequently no
> >   unpopulated event will be generated. The assumption is that kthreads
> >   should be in the root cgroup only anyway so this is not an issue.
> > - We skip killing tasks that already have pending fatal signals.
> > - Freezer doesn't care about tasks in different pid namespaces, i.e. if
> >   you have two tasks in different pid namespaces the cgroup would still
> >   be frozen. The cgroup.kill mechanism consequently behaves the same
> >   way, i.e. we kill all processes and ignore in which pid namespace they
> >   exist.
> > - If the caller is located in a cgroup that is killed the caller will
> >   obviously be killed as well.
> > 
> > Cc: Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>
> > Cc: Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>
> > Cc: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> > Cc: cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> > Signed-off-by: Christian Brauner <christian.brauner-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org>
> 
> Hello Christian!
> 
> This version looks very good to me, thanks for working on it!
> One small not below.
> 
> > ---
> >  include/linux/cgroup-defs.h |  3 ++
> >  kernel/cgroup/cgroup.c      | 90 +++++++++++++++++++++++++++++++++++++
> >  2 files changed, 93 insertions(+)
> > 
> > diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> > index 559ee05f86b2..43fef771009a 100644
> > --- a/include/linux/cgroup-defs.h
> > +++ b/include/linux/cgroup-defs.h
> > @@ -71,6 +71,9 @@ enum {
> >  
> >  	/* Cgroup is frozen. */
> >  	CGRP_FROZEN,
> > +
> > +	/* Control group has to be killed. */
> > +	CGRP_KILL,
> >  };
> >  
> >  /* cgroup_root->flags */
> > diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> > index 9153b20e5cc6..000f58b6ea35 100644
> > --- a/kernel/cgroup/cgroup.c
> > +++ b/kernel/cgroup/cgroup.c
> > @@ -3654,6 +3654,80 @@ static ssize_t cgroup_freeze_write(struct kernfs_open_file *of,
> >  	return nbytes;
> >  }
> >  
> > +static void __cgroup_kill(struct cgroup *cgrp)
> > +{
> > +	struct css_task_iter it;
> > +	struct task_struct *task;
> > +
> > +	lockdep_assert_held(&cgroup_mutex);
> > +
> > +	spin_lock_irq(&css_set_lock);
> > +	set_bit(CGRP_KILL, &cgrp->flags);
> > +	spin_unlock_irq(&css_set_lock);
> > +
> > +	css_task_iter_start(&cgrp->self, CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED, &it);
> > +	while ((task = css_task_iter_next(&it))) {
> > +		/* Ignore kernel threads here. */
> > +		if (task->flags & PF_KTHREAD)
> > +			continue;
> > +
> > +		/* Skip tasks that are already dying. */
> > +		if (__fatal_signal_pending(task))
> > +			continue;
> > +
> > +		send_sig(SIGKILL, task, 0);
> > +	}
> > +	css_task_iter_end(&it);
> > +
> > +	spin_lock_irq(&css_set_lock);
> > +	clear_bit(CGRP_KILL, &cgrp->flags);
> > +	spin_unlock_irq(&css_set_lock);
> > +}
> > +
> > +static void cgroup_kill(struct cgroup *cgrp)
> > +{
> > +	struct cgroup_subsys_state *css;
> > +	struct cgroup *dsct;
> > +
> > +	lockdep_assert_held(&cgroup_mutex);
> > +
> > +	cgroup_for_each_live_descendant_pre(dsct, css, cgrp)
> > +		__cgroup_kill(dsct);
> > +}
> > +
> > +static ssize_t cgroup_kill_write(struct kernfs_open_file *of, char *buf,
> > +				 size_t nbytes, loff_t off)
> > +{
> > +	ssize_t ret = 0;
> > +	int kill;
> > +	struct cgroup *cgrp;
> > +
> > +	ret = kstrtoint(strstrip(buf), 0, &kill);
> > +	if (ret)
> > +		return ret;
> > +
> > +	if (kill != 1)
> > +		return -ERANGE;
> > +
> > +	cgrp = cgroup_kn_lock_live(of->kn, false);
> > +	if (!cgrp)
> > +		return -ENOENT;
> > +
> > +	/*
> > +	 * Killing is a process directed operation, i.e. the whole thread-group
> > +	 * is taken down so act like we do for cgroup.procs and only make this
> > +	 * writable in non-threaded cgroups.
> > +	 */
> > +	if (cgroup_is_threaded(cgrp))
> > +		ret = -EOPNOTSUPP;
> > +	else
> > +		cgroup_kill(cgrp);
> > +
> > +	cgroup_kn_unlock(of->kn);
> > +
> > +	return ret ?: nbytes;
> > +}
> > +
> >  static int cgroup_file_open(struct kernfs_open_file *of)
> >  {
> >  	struct cftype *cft = of_cft(of);
> > @@ -4846,6 +4920,11 @@ static struct cftype cgroup_base_files[] = {
> >  		.seq_show = cgroup_freeze_show,
> >  		.write = cgroup_freeze_write,
> >  	},
> > +	{
> > +		.name = "cgroup.kill",
> > +		.flags = CFTYPE_NOT_ON_ROOT,
> > +		.write = cgroup_kill_write,
> > +	},
> >  	{
> >  		.name = "cpu.stat",
> >  		.seq_show = cpu_stat_show,
> > @@ -6077,6 +6156,7 @@ void cgroup_post_fork(struct task_struct *child,
> >  		      struct kernel_clone_args *kargs)
> >  	__releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
> >  {
> > +	bool kill = false;
> >  	struct cgroup_subsys *ss;
> >  	struct css_set *cset;
> >  	int i;
> > @@ -6088,6 +6168,12 @@ void cgroup_post_fork(struct task_struct *child,
> >  
> >  	/* init tasks are special, only link regular threads */
> >  	if (likely(child->pid)) {
> > +		struct cgroup *cgrp;
> > +
> > +		/* Check whether the target cgroup is being killed. */
> > +		cgrp = kargs->cgrp ?: cset->dfl_cgrp;
> > +		kill = test_bit(CGRP_KILL, &cgrp->flags);
> > +
> >  		WARN_ON_ONCE(!list_empty(&child->cg_list));
> >  		cset->nr_tasks++;
> >  		css_set_move_task(child, NULL, cset, false);
> > @@ -6135,6 +6221,10 @@ void cgroup_post_fork(struct task_struct *child,
> >  		put_css_set(rcset);
> >  	}
> >  
> > +	/* Take down child immediately. */
> > +	if (kill)
> > +		send_sig(SIGKILL, child, 0);
> > +
> 
> This part can be hot, so I'd integrate it better with the freezer check:
> instead of getting child task's cgroup's flags and checking individual
> bits twice we can do it once.

Oh, you essentially mean don't deref twice. Right. I'll tweak that.

Christian