linux-kernel - [PATCH] timekeeping: contribute wall clock to rng on time change

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Message-Id: <20220623165226.1335679-1-Jason@zx2c4.com>
Date:   Thu, 23 Jun 2022 18:52:26 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-crypto@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>, stable@...r.kernel.org
Subject: [PATCH] timekeeping: contribute wall clock to rng on time change

The rng's random_init() function contributes the real time to the rng at
boot time, so that events can at least start in relation to something
particular in the real world. But this clock might not yet be set that
point in boot, so nothing is contributed. In addition, the relation
between minor clock changes from, say, NTP, and the cycle counter is
potentially useful entropic data.

This commit addresses this by mixing in a time stamp on calls to
settimeofday and adjtimex. No entropy is credited in doing so, so it
doesn't make initialization faster, but it is still useful input to
have.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@...r.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 kernel/time/timekeeping.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 8e4b3c32fcf9..ad55da792f13 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1346,6 +1346,9 @@ int do_settimeofday64(const struct timespec64 *ts)
 	if (!ret)
 		audit_tk_injoffset(ts_delta);

+	ktime_get_real_ts64(&xt);
+	add_device_randomness(&xt, sizeof(xt));
+
 	return ret;
 }
 EXPORT_SYMBOL(do_settimeofday64);
@@ -2475,6 +2478,9 @@ int do_adjtimex(struct __kernel_timex *txc)

 	ntp_notify_cmos_timer();

+	ktime_get_real_ts64(&ts);
+	add_device_randomness(&ts, sizeof(ts));
+
 	return ret;
 }

-- 
2.35.1