linux-kernel - [RFC PATCH 05/41] random: don't reset entropy to zero on overflow

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Date:   Mon, 21 Sep 2020 09:58:21 +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 05/41] random: don't reset entropy to zero on overflow

credit_entropy_bits() adds one or more positive values to the signed
entropy_count and checks if the result is negative afterwards. Note that
because the initial value of entropy_count is positive, a negative result
can happen only on overflow.

However, if the final entropy_count is found to have overflown, a WARN()
is emitted and the entropy_store's entropy count reset to zero. Even
though this case should never happen, it is better to retain previously
available entropy as this will facilitate a future change factoring out
that approximation of the exponential.

Make credit_entropy_bits() tp reset entropy_count to the original value
rather than zero on overflow.

Signed-off-by: Nicolai Stange <nstange@...e.de>
---
 drivers/char/random.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 35e381be20fe..6adac462aa0d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -706,7 +706,7 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 	if (WARN_ON(entropy_count < 0)) {
 		pr_warn("negative entropy/overflow: pool %s count %d\n",
 			r->name, entropy_count);
-		entropy_count = 0;
+		entropy_count = orig;
 	} else if (entropy_count > pool_size)
 		entropy_count = pool_size;
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
-- 
2.26.2