Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 5 Feb 2021 13:00:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>, tglx@linutronix.de,
        linux-kernel@vger.kernel.org, mingo@kernel.org,
        torvalds@linux-foundation.org, fweisbec@gmail.com,
        keescook@chromium.org, kerrnel@google.com,
        Phil Auld <pauld@redhat.com>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, vineeth@bitbyteword.org,
        Chen Yu <yu.c.chen@intel.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Agata Gruza <agata.gruza@intel.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@intel.com>,
        graf@amazon.com, konrad.wilk@oracle.com, dfaggioli@suse.com,
        pjt@google.com, rostedt@goodmis.org, derkling@google.com,
        benbjiang@tencent.com,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        James.Bottomley@hansenpartnership.com, OWeisse@umich.edu,
        Dhaval Giani <dhaval.giani@oracle.com>,
        Junaid Shahid <junaids@google.com>, jsbarnes@google.com,
        chris.hyser@oracle.com, Ben Segall <bsegall@google.com>,
        Josh Don <joshdon@google.com>, Hao Luo <haoluo@google.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
Subject: Re: [PATCH v10 2/5] sched: CGroup tagging interface for core
 scheduling
Message-ID: <YB0zSwrAozzRxhFV@hirez.programming.kicks-ass.net>
References: <20210123011704.1901835-1-joel@joelfernandes.org>
 <20210123011704.1901835-3-joel@joelfernandes.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20210123011704.1901835-3-joel@joelfernandes.org>
Precedence: bulk

On Fri, Jan 22, 2021 at 08:17:01PM -0500, Joel Fernandes (Google) wrote:
> +int sched_core_share_tasks(struct task_struct *t1, struct task_struct *t2)
> +{
> +	static DEFINE_MUTEX(sched_core_tasks_mutex);
> +	unsigned long cookie;
> +	int ret = -ENOMEM;
> +
> +	mutex_lock(&sched_core_tasks_mutex);
> +
> +	if (!t2) {
> +		if (t1->core_task_cookie) {
> +			sched_core_put_task_cookie(t1->core_task_cookie);
> +			sched_core_update_task_cookie(t1, 0);
> +			sched_core_put();
> +		}

So this seems to be the bit that implements _CLEAR. ISTR there were
security implications / considerations here.

When the machine is vulnerable to L1TF/MDS and the like, clearing the
cookie would gain privilege and should thus be subject to some checks,
but I can'd find anything.

At the very least that deserves a comment I'm thinking.