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Date:   Mon, 8 Nov 2021 12:40:19 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mathias Krause <minipli@grsecurity.net>
Cc:     Vincent Guittot <vincent.guittot@linaro.org>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        Benjamin Segall <bsegall@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <Valentin.Schneider@arm.com>,
        linux-kernel@vger.kernel.org, Odin Ugedal <odin@uged.al>,
        Kevin Tanguy <kevin.tanguy@corp.ovh.com>,
        Brad Spengler <spender@grsecurity.net>
Subject: Re: [PATCH] sched/fair: Prevent dead task groups from regaining
 cfs_rq's
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On Mon, Nov 08, 2021 at 11:27:57AM +0100, Mathias Krause wrote:

> The timers need to be destroyed prior to unregister_fair_sched_group()
> via destroy_cfs_bandwidth(tg_cfs_bandwidth(tg)), i.e. move it from
> free_fair_sched_group() to here, as I did in my patch. Otherwise the tg
> might still be messed with and we don't want that.

Oh, duh, yes. For consistency's sake, I've also added an unregister_*
for the rt class, also destroying the bandwidth timer.

> Beside that, looks good to me. Will you create a new proper patch or
> should I do it?

Something like so good?

(I stripped the #PF splat, because I don't think it adds anything not
covered by the text).

---
Subject: sched/fair: Prevent dead task groups from regaining cfs_rq's
From: Mathias Krause <minipli@grsecurity.net>
Date: Wed, 3 Nov 2021 20:06:13 +0100

From: Mathias Krause <minipli@grsecurity.net>

Kevin is reporting crashes which point to a use-after-free of a cfs_rq
in update_blocked_averages(). Initial debugging revealed that we've
live cfs_rq's (on_list=1) in an about to be kfree()'d task group in
free_fair_sched_group(). However, it was unclear how that can happen.

His kernel config happened to lead to a layout of struct sched_entity
that put the 'my_q' member directly into the middle of the object
which makes it incidentally overlap with SLUB's freelist pointer.
That, in combination with SLAB_FREELIST_HARDENED's freelist pointer
mangling, leads to a reliable access violation in form of a #GP which
made the UAF fail fast.

Michal seems to have run into the same issue[1]. He already correctly
diagnosed that commit a7b359fc6a37 ("sched/fair: Correctly insert
cfs_rq's to list on unthrottle") is causing the preconditions for the
UAF to happen by re-adding cfs_rq's also to task groups that have no
more running tasks, i.e. also to dead ones. His analysis, however,
misses the real root cause and it cannot be seen from the crash
backtrace only, as the real offender is tg_unthrottle_up() getting
called via sched_cfs_period_timer() via the timer interrupt at an
inconvenient time.

When unregister_fair_sched_group() unlinks all cfs_rq's from the dying
task group, it doesn't protect itself from getting interrupted. If the
timer interrupt triggers while we iterate over all CPUs or after
unregister_fair_sched_group() has finished but prior to unlinking the
task group, sched_cfs_period_timer() will execute and walk the list of
task groups, trying to unthrottle cfs_rq's, i.e. re-add them to the
dying task group. These will later -- in free_fair_sched_group() -- be
kfree()'ed while still being linked, leading to the fireworks Kevin
and Michal are seeing.

To fix this race, ensure the dying task group gets unlinked first.
However, simply switching the order of unregistering and unlinking the
task group isn't sufficient, as concurrent RCU walkers might still see
it, as can be seen below:

    CPU1:                                      CPU2:
      :                                        timer IRQ:
      :                                          do_sched_cfs_period_timer():
      :                                            :
      :                                            distribute_cfs_runtime():
      :                                              rcu_read_lock();
      :                                              :
      :                                              unthrottle_cfs_rq():
    sched_offline_group():                             :
      :                                                walk_tg_tree_from(…,tg_unthrottle_up,…):
      list_del_rcu(&tg->list);                           :
 (1)  :                                                  list_for_each_entry_rcu(child, &parent->children, siblings)
      :                                                    :
 (2)  list_del_rcu(&tg->siblings);                         :
      :                                                    tg_unthrottle_up():
      unregister_fair_sched_group():                         struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
        :                                                    :
        list_del_leaf_cfs_rq(tg->cfs_rq[cpu]);               :
        :                                                    :
        :                                                    if (!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running)
 (3)    :                                                        list_add_leaf_cfs_rq(cfs_rq);
      :                                                      :
      :                                                    :
      :                                                  :
      :                                                :
      :                                              :
 (4)  :                                              rcu_read_unlock();

CPU 2 walks the task group list in parallel to sched_offline_group(),
specifically, it'll read the soon to be unlinked task group entry at
(1). Unlinking it on CPU 1 at (2) therefore won't prevent CPU 2 from
still passing it on to tg_unthrottle_up(). CPU 1 now tries to unlink
all cfs_rq's via list_del_leaf_cfs_rq() in
unregister_fair_sched_group().  Meanwhile CPU 2 will re-add some of
these at (3), which is the cause of the UAF later on.

To prevent this additional race from happening, we need to wait until
walk_tg_tree_from() has finished traversing the task groups, i.e.
after the RCU read critical section ends in (4). Afterwards we're safe
to call unregister_fair_sched_group(), as each new walk won't see the
dying task group any more.

Using synchronize_rcu() might be seen as a too heavy hammer to nail
this problem. However, the overall tear down sequence (e.g., as
documented in css_free_rwork_fn()) already relies on quite a few
assumptions regarding execution context and RCU grace periods from
passing. Looking at the autogroup code, which calls
sched_destroy_group() directly after sched_offline_group() and the
apparent need to have at least one RCU grace period expire after
unlinking the task group, prior to calling
unregister_fair_sched_group(), there seems to be no better
alternative.  Calling unregister_fair_sched_group() via call_rcu()
will only lead to trouble in sched_offline_group() which also relies
on (yet another) expired RCU grace period.

This patch survives Michal's reproducer[2] for 8h+ now, which used to
trigger within minutes before.

  [1] https://lore.kernel.org/lkml/20211011172236.11223-1-mkoutny@suse.com/
  [2] https://lore.kernel.org/lkml/20211102160228.GA57072@blackbody.suse.cz/

Fixes: a7b359fc6a37 ("sched/fair: Correctly insert cfs_rq's to list on unthrottle")
Reported-by: Kevin Tanguy <kevin.tanguy@corp.ovh.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
[peterz: shuffle code around a bit]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/autogroup.c |    2 +-
 kernel/sched/core.c      |   46 ++++++++++++++++++++++++++++++++++++----------
 kernel/sched/fair.c      |    4 ++--
 kernel/sched/rt.c        |   12 +++++++++---
 kernel/sched/sched.h     |    3 ++-
 5 files changed, 50 insertions(+), 17 deletions(-)

--- a/kernel/sched/autogroup.c
+++ b/kernel/sched/autogroup.c
@@ -31,7 +31,7 @@ static inline void autogroup_destroy(str
 	ag->tg->rt_se = NULL;
 	ag->tg->rt_rq = NULL;
 #endif
-	sched_offline_group(ag->tg);
+	sched_release_group(ag->tg);
 	sched_destroy_group(ag->tg);
 }
 
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -9716,6 +9716,22 @@ static void sched_free_group(struct task
 	kmem_cache_free(task_group_cache, tg);
 }
 
+static void sched_free_group_rcu(struct rcu_head *rcu)
+{
+	sched_free_group(container_of(rcu, struct task_group, rcu));
+}
+
+static void sched_unregister_group(struct task_group *tg)
+{
+	unregister_fair_sched_group(tg);
+	unregister_rt_sched_group(tg);
+	/*
+	 * We have to wait for yet another RCU grace period to expire, as
+	 * print_cfs_stats() might run concurrently.
+	 */
+	call_rcu(&tg->rcu, sched_free_group_rcu);
+}
+
 /* allocate runqueue etc for a new task group */
 struct task_group *sched_create_group(struct task_group *parent)
 {
@@ -9736,7 +9752,7 @@ struct task_group *sched_create_group(st
 	return tg;
 
 err:
-	sched_free_group(tg);
+	sched_unregister_group(tg);
 	return ERR_PTR(-ENOMEM);
 }
 
@@ -9759,25 +9775,35 @@ void sched_online_group(struct task_grou
 }
 
 /* rcu callback to free various structures associated with a task group */
-static void sched_free_group_rcu(struct rcu_head *rhp)
+static void sched_unregister_group_rcu(struct rcu_head *rhp)
 {
 	/* Now it should be safe to free those cfs_rqs: */
-	sched_free_group(container_of(rhp, struct task_group, rcu));
+	sched_unregister_group(container_of(rhp, struct task_group, rcu));
 }
 
 void sched_destroy_group(struct task_group *tg)
 {
 	/* Wait for possible concurrent references to cfs_rqs complete: */
-	call_rcu(&tg->rcu, sched_free_group_rcu);
+	call_rcu(&tg->rcu, sched_unregister_group_rcu);
 }
 
-void sched_offline_group(struct task_group *tg)
+void sched_release_group(struct task_group *tg)
 {
 	unsigned long flags;
 
-	/* End participation in shares distribution: */
-	unregister_fair_sched_group(tg);
-
+	/*
+	 * Unlink first, to avoid walk_tg_tree_from() from finding us (via
+	 * sched_cfs_period_timer()).
+	 *
+	 * For this to be effective, we have to wait for all pending users of
+	 * this task group to leave their RCU critical section to ensure no new
+	 * user will see our dying task group any more. Specifically ensure
+	 * that tg_unthrottle_up() won't add decayed cfs_rq's to it.
+	 *
+	 * We therefore defer calling unregister_fair_sched_group() to
+	 * sched_unregister_group() which is guarantied to get called only after the
+	 * current RCU grace period has expired.
+	 */
 	spin_lock_irqsave(&task_group_lock, flags);
 	list_del_rcu(&tg->list);
 	list_del_rcu(&tg->siblings);
@@ -9896,7 +9922,7 @@ static void cpu_cgroup_css_released(stru
 {
 	struct task_group *tg = css_tg(css);
 
-	sched_offline_group(tg);
+	sched_release_group(tg);
 }
 
 static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
@@ -9906,7 +9932,7 @@ static void cpu_cgroup_css_free(struct c
 	/*
 	 * Relies on the RCU grace period between css_released() and this.
 	 */
-	sched_free_group(tg);
+	sched_unregister_group(tg);
 }
 
 /*
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11456,8 +11456,6 @@ void free_fair_sched_group(struct task_g
 {
 	int i;
 
-	destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
-
 	for_each_possible_cpu(i) {
 		if (tg->cfs_rq)
 			kfree(tg->cfs_rq[i]);
@@ -11551,6 +11549,8 @@ void unregister_fair_sched_group(struct
 		list_del_leaf_cfs_rq(tg->cfs_rq[cpu]);
 		raw_spin_rq_unlock_irqrestore(rq, flags);
 	}
+
+	destroy_cfs_bandwidth(tg_cfs_bandwidth(tg));
 }
 
 void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -137,13 +137,17 @@ static inline struct rq *rq_of_rt_se(str
 	return rt_rq->rq;
 }
 
-void free_rt_sched_group(struct task_group *tg)
+void unregister_rt_sched_group(struct task_group *tg)
 {
-	int i;
-
 	if (tg->rt_se)
 		destroy_rt_bandwidth(&tg->rt_bandwidth);
 
+}
+
+void free_rt_sched_group(struct task_group *tg)
+{
+	int i;
+
 	for_each_possible_cpu(i) {
 		if (tg->rt_rq)
 			kfree(tg->rt_rq[i]);
@@ -250,6 +254,8 @@ static inline struct rt_rq *rt_rq_of_se(
 	return &rq->rt;
 }
 
+void unregister_rt_sched_group(struct task_group *tg) { }
+
 void free_rt_sched_group(struct task_group *tg) { }
 
 int alloc_rt_sched_group(struct task_group *tg, struct task_group *parent)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -488,6 +488,7 @@ extern void __refill_cfs_bandwidth_runti
 extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
 extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
 
+extern void unregister_rt_sched_group(struct task_group *tg);
 extern void free_rt_sched_group(struct task_group *tg);
 extern int alloc_rt_sched_group(struct task_group *tg, struct task_group *parent);
 extern void init_tg_rt_entry(struct task_group *tg, struct rt_rq *rt_rq,
@@ -503,7 +504,7 @@ extern struct task_group *sched_create_g
 extern void sched_online_group(struct task_group *tg,
 			       struct task_group *parent);
 extern void sched_destroy_group(struct task_group *tg);
-extern void sched_offline_group(struct task_group *tg);
+extern void sched_release_group(struct task_group *tg);
 
 extern void sched_move_task(struct task_struct *tsk);