linux-kernel - [PATCH] crypto: Jitter RNG - use ktime_get_raw

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Date:	Tue, 21 Jun 2016 20:05:59 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	herbert@...dor.apana.org.au
Cc:	John Stultz <john.stultz@...aro.org>,
	Arnd Bergmann <arnd@...db.de>,
	Alexander Kuleshov <kuleshovmail@...il.com>,
	y2038 Mailman List <y2038@...ts.linaro.org>,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Kees Cook <keescook@...omium.org>
Subject: [PATCH] crypto: Jitter RNG - use ktime_get_raw_ns as fallback

As part of the Y2038 development, __getnstimeofday is not supposed to be
used any more. It is now replaced with ktime_get_raw_ns. Albeit
ktime_get_raw_ns is monotonic compared to __getnstimeofday, this
difference is irrelevant as the Jitter RNG uses the time stamp to
measure the execution time of a given code path and tries to detect
variations in the execution time. Therefore, the only requirement the
Jitter RNG has, is a sufficient high resolution to detect these
variations.

The change was tested on x86 to show an identical behavior as RDTSC. The
used test code simply measures the execution time of the heart of the
RNG:

	jent_get_nstime(&time);
	jent_memaccess(ec, min);
	jent_fold_time(NULL, time, &folded, min);
	jent_get_nstime(&time2);
	return ((time2 - time));

Signed-off-by: Stephan Mueller <smueller@...onox.de>
---
 crypto/jitterentropy-kcapi.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/crypto/jitterentropy-kcapi.c b/crypto/jitterentropy-kcapi.c
index 597cedd..69a2988 100644
--- a/crypto/jitterentropy-kcapi.c
+++ b/crypto/jitterentropy-kcapi.c
@@ -87,24 +87,29 @@ void jent_memcpy(void *dest, const void *src, unsigned int n)
 	memcpy(dest, src, n);
 }
 
+/*
+ * Obtain a high-resolution time stamp value. The time stamp is used to measure
+ * the execution time of a given code path and its variations. Hence, the time
+ * stamp must have a sufficiently high resolution. It is valid if the time
+ * runs backwards for short period of time as the RNG code is able handle that.
+ *
+ * Note, if the function returns zero because a given architecture does not
+ * implement a high-resolution time stamp, the RNG code's runtime test
+ * will detect it and will not produce output.
+ */
 void jent_get_nstime(__u64 *out)
 {
-	struct timespec ts;
 	__u64 tmp = 0;
 
 	tmp = random_get_entropy();
 
 	/*
-	 * If random_get_entropy does not return a value (which is possible on,
-	 * for example, MIPS), invoke __getnstimeofday
+	 * If random_get_entropy does not return a value, i.e. it is not
+	 * implemented for a given architecture, invoke ktime_get_raw_ns
 	 * hoping that there are timers we can work with.
 	 */
-	if ((0 == tmp) &&
-	   (0 == __getnstimeofday(&ts))) {
-		tmp = ts.tv_sec;
-		tmp = tmp << 32;
-		tmp = tmp | ts.tv_nsec;
-	}
+	if (tmp == 0)
+		tmp = ktime_get_raw_ns();
 
 	*out = tmp;
 }
-- 
2.5.5