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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Dominik Brodowski <linux@dominikbrodowski.net>,
        Theodore Tso <tytso@mit.edu>,
        "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: [PATCH 4.19 126/234] random: mix build-time latent entropy into pool at init
Date:   Thu, 23 Jun 2022 18:43:13 +0200
Message-Id: <20220623164346.623027850@linuxfoundation.org>
In-Reply-To: <20220623164343.042598055@linuxfoundation.org>
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From: "Jason A. Donenfeld" <Jason@zx2c4.com>

commit 1754abb3e7583c570666fa1e1ee5b317e88c89a0 upstream.

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")
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/char/random.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -967,6 +967,11 @@ 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)) {