linux-kernel - [PATCH v8 4/7] random: mix bootloader randomness into pool

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Date:   Wed, 29 Dec 2021 22:10:06 +0100
From:   Dominik Brodowski <linux@...inikbrodowski.net>
To:     "Jason A . Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
        "Ivan T . Ivanov" <iivanov@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        linux@...inikbrodowski.net
Subject: [PATCH v8 4/7] random: mix bootloader randomness into pool

From: "Jason A. Donenfeld" <Jason@...c4.com>

If we're trusting bootloader randomness, crng_fast_load() is called by
add_hwgenerator_randomness(), which sets us to crng_init==1. However,
usually it is only called once for an initial 64-byte push, so bootloader
entropy will not mix any bytes into the input pool. So it's conceivable
that crng_init==1 when crng_initialize_primary() is called later, but
then the input pool is empty. When that happens, the crng state key will
be overwritten with extracted output from the empty input pool. That's
bad.

In contrast, if we're not trusting bootloader randomness, we call
crng_slow_load() *and* we call mix_pool_bytes(), so that later
crng_initialize_primary() isn't drawing on nothing.

In order to prevent crng_initialize_primary() from extracting an empty
pool, have the trusted bootloader case mirror that of the untrusted
bootloader case, mixing the input into the pool.

Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
[linux@...inikbrodowski.net: rewrite commit message]
Signed-off-by: Dominik Brodowski <linux@...inikbrodowski.net>
---
 drivers/char/random.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 21166188b7e1..9d4e1907e4b1 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2298,6 +2298,7 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,

 	if (unlikely(crng_init == 0)) {
 		size_t ret = crng_fast_load(buffer, count);
+		mix_pool_bytes(poolp, buffer, ret);
 		count -= ret;
 		buffer += ret;
 		if (!count || crng_init == 0)
-- 
2.34.1