Re: [Y2038] [PATCH] crypto: Jitter RNG - use ktime_get_raw_ns as fallback

On Tue, Jun 21, 2016 at 11:05 AM, Stephan Mueller smueller@chronox.de wrote:

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@chronox.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();
  

I don't see in the above an explanation of *why* you're using ktime_get_raw_ns() instead of ktime_get_ns().

Also the bit about time running backwards being ok is confusing since you're not using the "fast" accessor where that would be a risk.

thanks -john