random: spread out jitter callback to different CPUs

Rather than merely hoping that the callback gets called on another CPU,
arrange for that to actually happen, by round robining which CPU the
timer fires on. This way, on multiprocessor machines, we exacerbate
jitter by touching the same memory from multiple different cores.

There's a little bit of tricky bookkeeping involved here, because using
timer_setup_on_stack() + add_timer_on() + del_timer_sync() will result
in a use after free. See this sample code: <https://xn--4db.cc/xBdEiIKO/c>.

Instead, it's necessary to call [try_to_]del_timer_sync() before calling
add_timer_on(), so that the final call to del_timer_sync() at the end of
the function actually succeeds at making sure no handlers are running.

Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -53,6 +53,7 @@
 #include <linux/uaccess.h>
 #include <linux/suspend.h>
 #include <linux/siphash.h>
+#include <linux/sched/isolation.h>
 #include <crypto/chacha.h>
 #include <crypto/blake2s.h>
 #include <asm/processor.h>
@@ -1232,7 +1233,8 @@ void __cold rand_initialize_disk(struct gendisk *disk)
 struct entropy_timer_state {
 	unsigned long entropy;
 	struct timer_list timer;
-	unsigned int samples, samples_per_bit;
+	atomic_t samples;
+	unsigned int samples_per_bit;
 };
 
 /*
@@ -1250,10 +1252,8 @@ static void __cold entropy_timer(struct timer_list *timer)
 {
 	struct entropy_timer_state *state = container_of(timer, struct entropy_timer_state, timer);
 
-	if (++state->samples == state->samples_per_bit) {
+	if (atomic_inc_return(&state->samples) % state->samples_per_bit == 0)
 		credit_init_bits(1);
-		state->samples = 0;
-	}
 }
 
 /*
@@ -1266,6 +1266,7 @@ static void __cold try_to_generate_entropy(void)
 	struct entropy_timer_state stack;
 	unsigned int i, num_different = 0;
 	unsigned long last = random_get_entropy();
+	int cpu = -1;
 
 	for (i = 0; i < NUM_TRIAL_SAMPLES - 1; ++i) {
 		stack.entropy = random_get_entropy();
@@ -1277,19 +1278,54 @@ static void __cold try_to_generate_entropy(void)
 	if (stack.samples_per_bit > MAX_SAMPLES_PER_BIT)
 		return;
 
-	stack.samples = 0;
+	atomic_set(&stack.samples, 0);
 	timer_setup_on_stack(&stack.timer, entropy_timer, 0);
 	while (!crng_ready() && !signal_pending(current)) {
-		if (!timer_pending(&stack.timer))
-			mod_timer(&stack.timer, jiffies);
+		/*
+		 * Check !timer_pending() and then ensure that any previous callback has finished
+		 * executing by checking try_to_del_timer_sync(), before queueing the next one.
+		 */
+		if (!timer_pending(&stack.timer) && try_to_del_timer_sync(&stack.timer) >= 0) {
+			struct cpumask timer_cpus;
+			unsigned int num_cpus;
+
+			/*
+			 * Preemption must be disabled here, both to read the current CPU number
+			 * and to avoid scheduling a timer on a dead CPU.
+			 */
+			preempt_disable();
+
+			/* Only schedule callbacks on timer CPUs that are online. */
+			cpumask_and(&timer_cpus, housekeeping_cpumask(HK_TYPE_TIMER), cpu_online_mask);
+			num_cpus = cpumask_weight(&timer_cpus);
+			/* In very bizarre case of misconfiguration, fallback to all online. */
+			if (unlikely(num_cpus == 0)) {
+				timer_cpus = *cpu_online_mask;
+				num_cpus = cpumask_weight(&timer_cpus);
+			}
+
+			/* Basic CPU round-robin, which avoids the current CPU. */
+			do {
+				cpu = cpumask_next(cpu, &timer_cpus);
+				if (cpu == nr_cpumask_bits)
+					cpu = cpumask_first(&timer_cpus);
+			} while (cpu == smp_processor_id() && num_cpus > 1);
+
+			/* Expiring the timer at `jiffies` means it's the next tick. */
+			stack.timer.expires = jiffies;
+
+			add_timer_on(&stack.timer, cpu);
+
+			preempt_enable();
+		}
 		mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 		schedule();
 		stack.entropy = random_get_entropy();
 	}
+	mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 
 	del_timer_sync(&stack.timer);
 	destroy_timer_on_stack(&stack.timer);
-	mix_pool_bytes(&stack.entropy, sizeof(stack.entropy));
 }