oom_kill.c: futex: delay the OOM reaper to allow time for proper futex cleanup

The pthread struct is allocated on PRIVATE|ANONYMOUS memory [1] which
can be targeted by the oom reaper.  This mapping is used to store the
futex robust list head; the kernel does not keep a copy of the robust
list and instead references a userspace address to maintain the
robustness during a process death.

A race can occur between exit_mm and the oom reaper that allows the oom
reaper to free the memory of the futex robust list before the exit path
has handled the futex death:

    CPU1                               CPU2
    --------------------------------------------------------------------
    page_fault
    do_exit "signal"
    wake_oom_reaper
                                        oom_reaper
                                        oom_reap_task_mm (invalidates mm)
    exit_mm
    exit_mm_release
    futex_exit_release
    futex_cleanup
    exit_robust_list
    get_user (EFAULT- can't access memory)

If the get_user EFAULT's, the kernel will be unable to recover the
waiters on the robust_list, leaving userspace mutexes hung indefinitely.

Delay the OOM reaper, allowing more time for the exit path to perform
the futex cleanup.

Reproducer: https://gitlab.com/jsavitz/oom_futex_reproducer

Based on a patch by Michal Hocko.

Link: https://elixir.bootlin.com/glibc/glibc-2.35/source/nptl/allocatestack.c#L370 [1]
Link: https://lkml.kernel.org/r/20220414144042.677008-1-npache@redhat.com
Fixes: 2129258 ("mm: oom: let oom_reap_task and exit_mmap run concurrently")
Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
Co-developed-by: Joel Savitz <jsavitz@redhat.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Herton R. Krzesinski <herton@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Savitz <jsavitz@redhat.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: PacheNico

include/linux/sched.h

@@ -1443,6 +1443,7 @@ struct task_struct {
 	int				pagefault_disabled;
 #ifdef CONFIG_MMU
 	struct task_struct		*oom_reaper_list;
+	struct timer_list		oom_reaper_timer;
 #endif
 #ifdef CONFIG_VMAP_STACK
 	struct vm_struct		*stack_vm_area;

mm/oom_kill.c

@@ -632,7 +632,7 @@ static void oom_reap_task(struct task_struct *tsk)
 	 */
 	set_bit(MMF_OOM_SKIP, &mm->flags);
 
-	/* Drop a reference taken by wake_oom_reaper */
+	/* Drop a reference taken by queue_oom_reaper */
 	put_task_struct(tsk);
 }
 
@@ -644,12 +644,12 @@ static int oom_reaper(void *unused)
 		struct task_struct *tsk = NULL;
 
 		wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL);
-		spin_lock(&oom_reaper_lock);
+		spin_lock_irq(&oom_reaper_lock);
 		if (oom_reaper_list != NULL) {
 			tsk = oom_reaper_list;
 			oom_reaper_list = tsk->oom_reaper_list;
 		}
-		spin_unlock(&oom_reaper_lock);
+		spin_unlock_irq(&oom_reaper_lock);
 
 		if (tsk)
 			oom_reap_task(tsk);
@@ -658,30 +658,56 @@ static int oom_reaper(void *unused)
 	return 0;
 }
 
-static void wake_oom_reaper(struct task_struct *tsk)
+static void wake_oom_reaper(struct timer_list *timer)
 {
-	/* mm is already queued? */
-	if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags))
-		return;
+	struct task_struct *tsk = container_of(timer, struct task_struct,
+			oom_reaper_timer);
+	struct mm_struct *mm = tsk->signal->oom_mm;
+	unsigned long flags;
 
-	get_task_struct(tsk);
+	/* The victim managed to terminate on its own - see exit_mmap */
+	if (test_bit(MMF_OOM_SKIP, &mm->flags)) {
+		put_task_struct(tsk);
+		return;
+	}
 
-	spin_lock(&oom_reaper_lock);
+	spin_lock_irqsave(&oom_reaper_lock, flags);
 	tsk->oom_reaper_list = oom_reaper_list;
 	oom_reaper_list = tsk;
-	spin_unlock(&oom_reaper_lock);
+	spin_unlock_irqrestore(&oom_reaper_lock, flags);
 	trace_wake_reaper(tsk->pid);
 	wake_up(&oom_reaper_wait);
 }
 
+/*
+ * Give the OOM victim time to exit naturally before invoking the oom_reaping.
+ * The timers timeout is arbitrary... the longer it is, the longer the worst
+ * case scenario for the OOM can take. If it is too small, the oom_reaper can
+ * get in the way and release resources needed by the process exit path.
+ * e.g. The futex robust list can sit in Anon|Private memory that gets reaped
+ * before the exit path is able to wake the futex waiters.
+ */
+#define OOM_REAPER_DELAY (2*HZ)
+static void queue_oom_reaper(struct task_struct *tsk)
+{
+	/* mm is already queued? */
+	if (test_and_set_bit(MMF_OOM_REAP_QUEUED, &tsk->signal->oom_mm->flags))
+		return;
+
+	get_task_struct(tsk);
+	timer_setup(&tsk->oom_reaper_timer, wake_oom_reaper, 0);
+	tsk->oom_reaper_timer.expires = jiffies + OOM_REAPER_DELAY;
+	add_timer(&tsk->oom_reaper_timer);
+}
+
 static int __init oom_init(void)
 {
 	oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper");
 	return 0;
 }
 subsys_initcall(oom_init)
 #else
-static inline void wake_oom_reaper(struct task_struct *tsk)
+static inline void queue_oom_reaper(struct task_struct *tsk)
 {
 }
 #endif /* CONFIG_MMU */
@@ -932,7 +958,7 @@ static void __oom_kill_process(struct task_struct *victim, const char *message)
 	rcu_read_unlock();
 
 	if (can_oom_reap)
-		wake_oom_reaper(victim);
+		queue_oom_reaper(victim);
 
 	mmdrop(mm);
 	put_task_struct(victim);
@@ -968,7 +994,7 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
 	task_lock(victim);
 	if (task_will_free_mem(victim)) {
 		mark_oom_victim(victim);
-		wake_oom_reaper(victim);
+		queue_oom_reaper(victim);
 		task_unlock(victim);
 		put_task_struct(victim);
 		return;
@@ -1067,7 +1093,7 @@ bool out_of_memory(struct oom_control *oc)
 	 */
 	if (task_will_free_mem(current)) {
 		mark_oom_victim(current);
-		wake_oom_reaper(current);
+		queue_oom_reaper(current);
 		return true;
 	}