linux-kernel - Re: [PATCH v2] random: avoid superfluous call to RDRAND in CRNG extraction

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Message-ID: <CACXcFmm2nKLHdqN27Ced2nLg=h2mSX_fKWFf-OkgArVRDi3xTw@mail.gmail.com>
Date:   Tue, 4 Jan 2022 13:03:43 +0800
From:   Sandy Harris <sandyinchina@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>
Subject: Re: [PATCH v2] random: avoid superfluous call to RDRAND in CRNG extraction

If we are removing RDRAND, what about adding some
cheaper mixing? Something along these lines?

The current code's mixing is triggered only once in 2^32
iterations, depends only on crng->state[], always changes
the same state word, and introduces no new entropy.

Make it happen more often, depend on a randomly initialised
counter as well as state[], make a data-dependent choice of
word to change, and use random_get_entropy().
---
 drivers/char/random.c | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 605969ed0f96..d2be079f004d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -985,6 +985,10 @@ static void crng_reseed(struct crng_state *crng,
struct entropy_store *r)
     }
 }

+#define CC_SHIFT 8
+#define CC_MASK ((1<<CC_SHIFT)-1)
+static u32 cc_count = 0 ;
+
 static void _extract_crng(struct crng_state *crng,
               __u8 out[CHACHA_BLOCK_SIZE])
 {
@@ -998,8 +1002,22 @@ static void _extract_crng(struct crng_state *crng,
     if (arch_get_random_long(&v))
         crng->state[14] ^= v;
     chacha20_block(&crng->state[0], out);
-    if (crng->state[12] == 0)
-        crng->state[13]++;
+        if (cc_count == 0)
+                cc_count = crng->state[9] ^ random_get_entropy() ;
+    switch ((crng->state[12] ^ cc_count) & CC_MASK)        {
+                case 0:
+                        cc_count = crng->state[10] ^ (cc_count>>CC_SHIFT);
+                        break ;
+                case 31: case 97: case 253:
+                        crng->state[crng->state[13]&7]++;
+                        break ;
+                case 61: case 127:
+                        crng->state[crng->state[11]&7] += random_get_entropy();
+                        break ;
+                default:
+                        break ;
+        }
+        cc_count++ ;
     spin_unlock_irqrestore(&crng->lock, flags);
 }

-- 
Signed-off-by: Sandy Harris <sandyinchina@...il.com>