linux-kernel - [RFC PATCH 28/41] random: don't award entropy to disk + input events if in FIPS mode

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Date:   Mon, 21 Sep 2020 09:58:44 +0200
From:   Nicolai Stange <nstange@...e.de>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Vito Caputo <vcaputo@...garu.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.cz>, Ray Strode <rstrode@...hat.com>,
        William Jon McCann <mccann@....edu>,
        zhangjs <zachary@...shancloud.com>,
        Andy Lutomirski <luto@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        Peter Matthias <matthias.peter@....bund.de>,
        Marcelo Henrique Cerri <marcelo.cerri@...onical.com>,
        Roman Drahtmueller <draht@...altsekun.de>,
        Neil Horman <nhorman@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andy Lavr <andy.lavr@...il.com>,
        Eric Biggers <ebiggers@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Stephan Müller <smueller@...onox.de>,
        Torsten Duwe <duwe@...e.de>, Petr Tesarik <ptesarik@...e.cz>,
        Nicolai Stange <nstange@...e.de>
Subject: [RFC PATCH 28/41] random: don't award entropy to disk + input events if in FIPS mode

NIST SP800-90C prohibits the use of multiple correlated entropy sources.

Obviously, add_disk_randomness(), add_input_randomness() and
add_interrupt_randomness() are not independent.

Follow the approach taken by Stephan Müller's LRNG patchset ([1]) and don't
award any entropy to the former two if fips_enabled is true.
Note that the entropy loss has already been compensated for by a previous
patch increasing the IRQ event estimate.

The actual entropy accounting from add_disk_randomness() and
add_input_randomness() is implemented in the common add_timer_randomness()
called therefrom.

Make the latter to not dispatch any entropy to the global entropy balance
if fips_enabled is on.

[1] https://lkml.kernel.org/r/5695397.lOV4Wx5bFT@positron.chronox.de

Suggested-by: Stephan Müller <smueller@...onox.de>
Signed-off-by: Nicolai Stange <nstange@...e.de>
---
 drivers/char/random.c | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 8f79e90f2429..680ccc82a436 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1481,12 +1481,24 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 
 	r = &input_pool;
 	spin_lock_irqsave(&r->lock, flags);
-	/*
-	 * delta is now minimum absolute delta.
-	 * Round down by 1 bit on general principles,
-	 * and limit entropy estimate to 12 bits.
-	 */
-	__queue_entropy(r, &q, min_t(int, fls(delta>>1), 11) << ENTROPY_SHIFT);
+	if (!fips_enabled) {
+		unsigned int nfrac;
+
+		/*
+		 * delta is now minimum absolute delta.
+		 * Round down by 1 bit on general principles,
+		 * and limit entropy estimate to 12 bits.
+		 */
+		nfrac = min_t(int, fls(delta>>1), 11) << ENTROPY_SHIFT;
+		__queue_entropy(r, &q, nfrac);
+	} else {
+		/*
+		 * Multiple correlated entropy sources are prohibited
+		 * by NIST SP800-90C. Leave it up to
+		 * add_interrupt_randomness() to contribute any
+		 * entropy.
+		 */
+	}
 	__mix_pool_bytes(r, &sample, sizeof(sample));
 	reseed = __dispatch_queued_entropy_fast(r, &q);
 	spin_unlock_irqrestore(&r->lock, flags);
-- 
2.26.2