[PATCH v4 2/2] random: invalidate crngs and batches in cpuhp prepare

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Now that we have a cpuhp prepare notifier, we can invalidate the keys
used by the per-cpu crngs and the batches used by per-cpu batched
entropy, so that if the cpus come back online, and the generation
counter happens to have cycled all the way around to where it was
before, it doesn't mistakenly use the old data. The chances of this
happening are exceedingly rare, but since we now have the notifier
setup, doing this is basically free.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Sultan Alsawaf <sultan@kerneltoast.com>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 drivers/char/random.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index a3cc147406b0..41188a49d43e 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1233,6 +1233,14 @@ int random_prepare_cpu(unsigned int cpu)
 	 * since the MIX_INFLIGHT flag will be cleared.
 	 */
 	per_cpu_ptr(&irq_randomness, cpu)->count = 0;
+
+	/*
+	 * We also want to invalidate per-cpu crngs and batches, so
+	 * that we always use fresh entropy.
+	 */
+	per_cpu_ptr(&crngs, cpu)->generation = ULONG_MAX;
+	per_cpu_ptr(&batched_entropy_u32, cpu)->position = UINT_MAX;
+	per_cpu_ptr(&batched_entropy_u64, cpu)->position = UINT_MAX;
 	return 0;
 }
 #endif
-- 
2.35.0