git: 39c32c5bca84 - stable/12 - random/ivy: Provide mechanism to read independent seed values from rdrand

The branch stable/12 has been updated by obrien:

URL: https://cgit.FreeBSD.org/src/commit/?id=39c32c5bca849b022bc3eb68726bdc1388193c75

commit 39c32c5bca849b022bc3eb68726bdc1388193c75
Author:     Conrad Meyer <cem@FreeBSD.org>
AuthorDate: 2019-11-22 19:30:31 +0000
Commit:     David E. O'Brien <obrien@FreeBSD.org>
CommitDate: 2022-02-21 05:56:43 +0000

    random/ivy: Provide mechanism to read independent seed values from rdrand
    
    On x86 platforms with the intrinsic, rdrand is a deterministic bit generator
    (AES-CTR) seeded from an entropic source.  On x86 platforms with rdseed, it
    is something closer to the upstream entropic source.  (There is more nuance;
    a block diagram is provided in [1].)
    
    On devices with rdrand and without rdseed, there is no good intrinsic for
    acecssing the good entropic soure directly.  However, the DRBG is guaranteed
    to reseed every 8 kB on these platforms.  As a conservative option, on such
    hardware we can read an extra 7.99kB samples every time we want a sample
    from an independent seed.
    
    As one can imagine, this drastically slows the effective read rate of
    RDRAND (a factor of 1024 on amd64 and 2048 on ia32).  Microbenchmarks on AMD
    Zen (has RDSEED) show an RDRAND rate of 25 MB/s and Intel Haswell (no
    RDSEED) show RDRAND of 170 MB/s.  This would reduce the read rate on Haswell
    to ~170 kB/s (at 100% CPU).  random(4)'s harvestq thread periodically
    "feeds" from pure sources in amounts of 128-1024 bytes.  On Haswell,
    enabling this feature increases the CPU time of RDRAND in each "feed" from
    approximately 0.7-6 µs to 0.7-6 ms.
    
    Because there is some performance penalty to this more conservative option,
    a knob is provided to enable the change.  The change does not affect
    platforms with RDSEED.
    
    [1]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide#inpage-nav-4-2
    
    (cherry picked from commit cb285f7c7c5ed93ca01632e839d8d093cb261a9e)
---
 sys/dev/random/ivy.c | 46 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 35 insertions(+), 11 deletions(-)

diff --git a/sys/dev/random/ivy.c b/sys/dev/random/ivy.c
index a33e6bb63327..dfb477fe59fb 100644
--- a/sys/dev/random/ivy.c
+++ b/sys/dev/random/ivy.c
@@ -40,6 +40,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/malloc.h>
 #include <sys/module.h>
 #include <sys/random.h>
+#include <sys/sysctl.h>
 #include <sys/systm.h>
 
 #include <machine/md_var.h>
@@ -59,23 +60,46 @@ static struct random_source random_ivy = {
 	.rs_read = random_ivy_read
 };
 
+SYSCTL_NODE(_kern_random, OID_AUTO, rdrand, CTLFLAG_RW, 0,
+    "rdrand (ivy) entropy source");
+static bool acquire_independent_seed_samples = false;
+SYSCTL_BOOL(_kern_random_rdrand, OID_AUTO, rdrand_independent_seed,
+    CTLFLAG_RWTUN, &acquire_independent_seed_samples, 0,
+    "If non-zero, use more expensive and slow, but safer, seeded samples "
+    "where RDSEED is not present.");
+
 static bool
 x86_rdrand_store(u_long *buf)
 {
-	u_long rndval;
+	u_long rndval, seed_iterations, i;
 	int retry;
 
-	retry = RETRY_COUNT;
-	__asm __volatile(
-	    "1:\n\t"
-	    "rdrand	%1\n\t"	/* read randomness into rndval */
-	    "jc		2f\n\t" /* CF is set on success, exit retry loop */
-	    "dec	%0\n\t" /* otherwise, retry-- */
-	    "jne	1b\n\t" /* and loop if retries are not exhausted */
-	    "2:"
-	    : "+r" (retry), "=r" (rndval) : : "cc");
+	/* Per [1], "§ 5.2.6 Generating Seeds from RDRAND,"
+	 * machines lacking RDSEED will guarantee RDRAND is reseeded every 8kB
+	 * of generated output.
+	 *
+	 * [1]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide#inpage-nav-6-8
+	 */
+	if (acquire_independent_seed_samples)
+		seed_iterations = 8 * 1024 / sizeof(*buf);
+	else
+		seed_iterations = 1;
+
+	for (i = 0; i < seed_iterations; i++) {
+		retry = RETRY_COUNT;
+		__asm __volatile(
+		    "1:\n\t"
+		    "rdrand	%1\n\t"	/* read randomness into rndval */
+		    "jc		2f\n\t" /* CF is set on success, exit retry loop */
+		    "dec	%0\n\t" /* otherwise, retry-- */
+		    "jne	1b\n\t" /* and loop if retries are not exhausted */
+		    "2:"
+		    : "+r" (retry), "=r" (rndval) : : "cc");
+		if (retry == 0)
+			return (false);
+	}
 	*buf = rndval;
-	return (retry != 0);
+	return (true);
 }
 
 static bool