linux-kernel - [PATCH] random: invalidate crngs and batches on cpuhp teardown

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Message-Id: <20220214134838.980159-1-Jason@zx2c4.com>
Date:   Mon, 14 Feb 2022 14:48:38 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux@...inikbrodowski.net
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Sultan Alsawaf <sultan@...neltoast.com>,
        Theodore Ts'o <tytso@....edu>
Subject: [PATCH] random: invalidate crngs and batches on cpuhp teardown

Now that we have a cpuhp teardown 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@...utronix.de>
Cc: Sultan Alsawaf <sultan@...neltoast.com>
Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index df5aef93da34..ce199af9bc56 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1225,6 +1225,15 @@ int random_dead_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 if the CPU does come back online, it uses
+	 * 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;
 }
 
-- 
2.35.0