linux-kernel - [PATCH v2] random: mix build-time latent entropy into pool at init

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Date:   Thu, 31 Mar 2022 11:26:41 -0400
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>
Subject: [PATCH v2] random: mix build-time latent entropy into pool at init

Prior, the "input_pool_data" array needed no real initialization, and so
it was easy to mark it with __latent_entropy to populate it during
compile-time. In switching to using a hash function, this required us to
specifically initialize it to some specific state, which means we
dropped the __latent_entropy attribute. An unfortunate side effect was
this meant the pool was no longer seeded using compile-time random data.
In order to bring this back, we declare an array in rand_initialize()
with __latent_entropy and call mix_pool_bytes() on that at init, which
accomplishes the same thing as before. We make this __initconst, so that
it doesn't take up space at runtime after init.

Fixes: 6e8ec2552c7d ("random: use computational hash for entropy extraction")
Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
Changes v1->v2:
- Use __initconst.

 drivers/char/random.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 40107f8b9e9e..1d8242969751 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -975,6 +975,11 @@ int __init rand_initialize(void)
 	bool arch_init = true;
 	unsigned long rv;

+#if defined(LATENT_ENTROPY_PLUGIN)
+	static const u8 compiletime_seed[BLAKE2S_BLOCK_SIZE] __initconst __latent_entropy;
+	_mix_pool_bytes(compiletime_seed, sizeof(compiletime_seed));
+#endif
+
 	for (i = 0; i < BLAKE2S_BLOCK_SIZE; i += sizeof(rv)) {
 		if (!arch_get_random_seed_long_early(&rv) &&
 		    !arch_get_random_long_early(&rv)) {
-- 
2.35.1