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Date:   Mon, 10 Aug 2020 13:47:00 +0200
From:   Willy Tarreau <w@....eu>
To:     George Spelvin <lkml@....org>
Cc:     netdev@...r.kernel.org, aksecurity@...il.com,
        torvalds@...ux-foundation.org, edumazet@...gle.com,
        Jason@...c4.com, luto@...nel.org, keescook@...omium.org,
        tglx@...utronix.de, peterz@...radead.org, tytso@....edu,
        lkml.mplumb@...il.com, stephen@...workplumber.org, fw@...len.de
Subject: Re: [DRAFT PATCH] random32: make prandom_u32() output unpredictable

Hi George,

On Sun, Aug 09, 2020 at 06:30:17PM +0000, George Spelvin wrote:
> Even something simple like buffering 8 TSC samples, and adding them
> at 32-bit offsets across the state every 8th call, would make a huge
> difference.

Doing testing on real hardware showed that retrieving the TSC on every
call had a non negligible cost, causing a loss of 2.5% on the accept()
rate and 4% on packet rate when using iptables -m statistics. However
I reused your idea of accumulating old TSCs to increase the uncertainty
about their exact value, except that I retrieve it only on 1/8 calls
and use the previous noise in this case. With this I observe the same
performance as plain 5.8. Below are the connection rates accepted on
a single core :

        5.8           5.8+patch     5.8+patch+tsc
   192900-197900   188800->192200   194500-197500  (conn/s)

This was on a core i7-8700K. I looked at the asm code for the function
and it remains reasonably light, in the same order of complexity as the
original one, so I think we could go with that.

My proposed change is below, in case you have any improvements to suggest.

Regards,
Willy


diff --git a/lib/random32.c b/lib/random32.c
index 2b048e2ea99f..a12d63028106 100644
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -317,6 +317,8 @@ static void __init prandom_state_selftest(void)
 
 struct siprand_state {
 	unsigned long v[4];
+	unsigned long noise;
+	unsigned long count;
 };
 
 static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy;
@@ -334,7 +336,7 @@ static DEFINE_PER_CPU(struct siprand_state, net_rand_state) __latent_entropy;
 #define K0 (0x736f6d6570736575 ^ 0x6c7967656e657261 )
 #define K1 (0x646f72616e646f6d ^ 0x7465646279746573 )
 
-#elif BITS_PER_LONG == 23
+#elif BITS_PER_LONG == 32
 /*
  * On 32-bit machines, we use HSipHash, a reduced-width version of SipHash.
  * This is weaker, but 32-bit machines are not used for high-traffic
@@ -375,6 +377,12 @@ static u32 siprand_u32(struct siprand_state *s)
 {
 	unsigned long v0 = s->v[0], v1 = s->v[1], v2 = s->v[2], v3 = s->v[3];
 
+	if (++s->count >= 8) {
+		v3 ^= s->noise;
+		s->noise += random_get_entropy();
+		s->count = 0;
+	}
+
 	SIPROUND(v0, v1, v2, v3);
 	SIPROUND(v0, v1, v2, v3);
 	s->v[0] = v0;  s->v[1] = v1;  s->v[2] = v2;  s->v[3] = v3;

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