hrtimer: Don't dereference the hrtimer pointer after the callback

A hrtimer can be released in its callback, but lockdep_hrtimer_exit()
dereferences the pointer after the callback returns, i.e. a potential use
after free.

Retrieve the context in which the hrtimer expires before the callback is
invoked and use it in lockdep_hrtimer_exit().

Fixes: 40db173965 ("lockdep: Add hrtimer context tracing bits")
Reported-by: syzbot+62c155c276e580cfb606@syzkaller.appspotmail.com
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200331201849.fkp2siy3vcdqvqlz@linutronix.de

include/linux/irqflags.h

@@ -58,16 +58,21 @@ do {						\
 } while (0)
 
 # define lockdep_hrtimer_enter(__hrtimer)		\
-	  do {						\
-		  if (!__hrtimer->is_hard)		\
-			current->irq_config = 1;	\
-	  } while (0)
+({							\
+	bool __expires_hardirq = true;			\
+							\
+	if (!__hrtimer->is_hard) {			\
+		current->irq_config = 1;		\
+		__expires_hardirq = false;		\
+	}						\
+	__expires_hardirq;				\
+})
 
-# define lockdep_hrtimer_exit(__hrtimer)		\
-	  do {						\
-		  if (!__hrtimer->is_hard)		\
+# define lockdep_hrtimer_exit(__expires_hardirq)	\
+	do {						\
+		if (!__expires_hardirq)			\
 			current->irq_config = 0;	\
-	  } while (0)
+	} while (0)
 
 # define lockdep_posixtimer_enter()				\
 	  do {							\
@@ -102,8 +107,8 @@ do {						\
 # define lockdep_hardirq_exit()		do { } while (0)
 # define lockdep_softirq_enter()	do { } while (0)
 # define lockdep_softirq_exit()		do { } while (0)
-# define lockdep_hrtimer_enter(__hrtimer)		do { } while (0)
-# define lockdep_hrtimer_exit(__hrtimer)		do { } while (0)
+# define lockdep_hrtimer_enter(__hrtimer)	false
+# define lockdep_hrtimer_exit(__context)	do { } while (0)
 # define lockdep_posixtimer_enter()		do { } while (0)
 # define lockdep_posixtimer_exit()		do { } while (0)
 # define lockdep_irq_work_enter(__work)		do { } while (0)

kernel/time/hrtimer.c

@@ -1480,6 +1480,7 @@ static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
 			  unsigned long flags) __must_hold(&cpu_base->lock)
 {
 	enum hrtimer_restart (*fn)(struct hrtimer *);
+	bool expires_in_hardirq;
 	int restart;
 
 	lockdep_assert_held(&cpu_base->lock);
@@ -1514,11 +1515,11 @@ static void __run_hrtimer(struct hrtimer_cpu_base *cpu_base,
 	 */
 	raw_spin_unlock_irqrestore(&cpu_base->lock, flags);
 	trace_hrtimer_expire_entry(timer, now);
-	lockdep_hrtimer_enter(timer);
+	expires_in_hardirq = lockdep_hrtimer_enter(timer);
 
 	restart = fn(timer);
 
-	lockdep_hrtimer_exit(timer);
+	lockdep_hrtimer_exit(expires_in_hardirq);
 	trace_hrtimer_expire_exit(timer);
 	raw_spin_lock_irq(&cpu_base->lock);