linux-kernel - Re: [PATCH] cgroup: always put cset in cgroup_css_set_put

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Message-ID: <20230523-aphorismen-erzwingen-ae57f4fd33f2@brauner>
Date:   Tue, 23 May 2023 11:58:35 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     John Sperbeck <jsperbeck@...gle.com>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup: always put cset in cgroup_css_set_put_fork

On Sun, May 21, 2023 at 07:29:53PM +0000, John Sperbeck wrote:
> A successful call to cgroup_css_set_fork() will always have taken
> a ref on kargs->cset (regardless of CLONE_INTO_CGROUP), so always
> do a corresponding put in cgroup_css_set_put_fork().
> 
> Without this, a cset and its contained css structures will be
> leaked for some fork failures.  The following script reproduces
> the leak for a fork failure due to exceeding pids.max in the
> pids controller.  A similar thing can happen if we jump to the
> bad_fork_cancel_cgroup label in copy_process().
> 
> [ -z "$1" ] && echo "Usage $0 pids-root" && exit 1
> PID_ROOT=$1
> CGROUP=$PID_ROOT/foo
> 
> [ -e $CGROUP ] && rmdir -f $CGROUP
> mkdir $CGROUP
> echo 5 > $CGROUP/pids.max
> echo $$ > $CGROUP/cgroup.procs
> 
> fork_bomb()
> {
> 	set -e
> 	for i in $(seq 10); do
> 		/bin/sleep 3600 &
> 	done
> }
> 
> (fork_bomb) &
> wait
> echo $$ > $PID_ROOT/cgroup.procs
> kill $(cat $CGROUP/cgroup.procs)
> rmdir $CGROUP
> 
> Fixes: ef2c41cf38a7 ("clone3: allow spawning processes into cgroups")
> Signed-off-by: John Sperbeck <jsperbeck@...gle.com>
> ---

Reviewed-by: Christian Brauner <brauner@...nel.org>