Re: [PATCH v2 1/5] cgroup: introduce cgroup.kill

["Serge E. Hallyn" <serge@hallyn.com>]

On Mon, May 03, 2021 at 04:39:19PM +0200, Christian Brauner wrote:
> From: Christian Brauner <christian.brauner@ubuntu.com>
> 
> 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@google.com>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: cgroups@vger.kernel.org
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

Reviewed-by: Serge Hallyn <serge@hallyn.com>

> ---
> 
> The series can be pulled from
> 
> git@gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux tags/cgroup.kill.v5.14
> 
> /* v2 */
> - Roman Gushchin <guro@fb.com>:
>   - Retrieve cgrp->flags only once and check CGRP_* bits on it.
> ---
>  include/linux/cgroup-defs.h |   3 +
>  kernel/cgroup/cgroup.c      | 127 ++++++++++++++++++++++++++++++++----
>  2 files changed, 116 insertions(+), 14 deletions(-)
> 
> 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..aee84b99534a 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 +6156,8 @@ void cgroup_post_fork(struct task_struct *child,
>  		      struct kernel_clone_args *kargs)
>  	__releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
>  {
> +	unsigned long cgrp_flags = 0;
> +	bool kill = false;
>  	struct cgroup_subsys *ss;
>  	struct css_set *cset;
>  	int i;
> @@ -6088,6 +6169,11 @@ void cgroup_post_fork(struct task_struct *child,
>  
>  	/* init tasks are special, only link regular threads */
>  	if (likely(child->pid)) {
> +		if (kargs->cgrp)
> +			cgrp_flags = kargs->cgrp->flags;
> +		else
> +			cgrp_flags = cset->dfl_cgrp->flags;
> +
>  		WARN_ON_ONCE(!list_empty(&child->cg_list));
>  		cset->nr_tasks++;
>  		css_set_move_task(child, NULL, cset, false);
> @@ -6096,23 +6182,32 @@ void cgroup_post_fork(struct task_struct *child,
>  		cset = NULL;
>  	}
>  
> -	/*
> -	 * If the cgroup has to be frozen, the new task has too.  Let's set
> -	 * the JOBCTL_TRAP_FREEZE jobctl bit to get the task into the
> -	 * frozen state.
> -	 */
> -	if (unlikely(cgroup_task_freeze(child))) {
> -		spin_lock(&child->sighand->siglock);
> -		WARN_ON_ONCE(child->frozen);
> -		child->jobctl |= JOBCTL_TRAP_FREEZE;
> -		spin_unlock(&child->sighand->siglock);
> +	if (!(child->flags & PF_KTHREAD)) {
> +		if (test_bit(CGRP_FREEZE, &cgrp_flags)) {
> +			/*
> +			 * If the cgroup has to be frozen, the new task has
> +			 * too. Let's set the JOBCTL_TRAP_FREEZE jobctl bit to
> +			 * get the task into the frozen state.
> +			 */
> +			spin_lock(&child->sighand->siglock);
> +			WARN_ON_ONCE(child->frozen);
> +			child->jobctl |= JOBCTL_TRAP_FREEZE;
> +			spin_unlock(&child->sighand->siglock);
> +
> +			/*
> +			 * Calling cgroup_update_frozen() isn't required here,
> +			 * because it will be called anyway a bit later from
> +			 * do_freezer_trap(). So we avoid cgroup's transient
> +			 * switch from the frozen state and back.
> +			 */
> +		}
>  
>  		/*
> -		 * Calling cgroup_update_frozen() isn't required here,
> -		 * because it will be called anyway a bit later from
> -		 * do_freezer_trap(). So we avoid cgroup's transient switch
> -		 * from the frozen state and back.
> +		 * If the cgroup is to be killed notice it now and take the
> +		 * child down right after we finished preparing it for
> +		 * userspace.
>  		 */
> +		kill = test_bit(CGRP_KILL, &cgrp_flags);
>  	}
>  
>  	spin_unlock_irq(&css_set_lock);
> @@ -6135,6 +6230,10 @@ void cgroup_post_fork(struct task_struct *child,
>  		put_css_set(rcset);
>  	}
>  
> +	/* Cgroup has to be killed so take down child immediately. */
> +	if (kill)
> +		send_sig(SIGKILL, child, 0);
> +
>  	cgroup_css_set_put_fork(kargs);
>  }
>  
> 
> base-commit: 9f4ad9e425a1d3b6a34617b8ea226d56a119a717
> -- 
> 2.27.0
>