[PATCH v2] random: avoid mis-detecting a slow counter as a cycle counter

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

The method that try_to_generate_entropy() uses to detect a cycle counter
is to check whether two calls to random_get_entropy() return different
values.  This is uncomfortably prone to false positives if
random_get_entropy() is a slow counter, as the two calls could return
different values if the counter happens to be on the cusp of a change.
Making things worse, the task can be preempted between the calls.

This is problematic because try_to_generate_entropy() doesn't do any
real entropy estimation later; it always credits 1 bit per loop
iteration.  To avoid crediting garbage, it relies entirely on the
preceding check for whether a cycle counter is present.

Therefore, increase the number of counter comparisons from 1 to 3, to
greatly reduce the rate of false positive cycle counter detections.

Fixes: 50ee7529ec45 ("random: try to actively add entropy rather than passively wait for it")
Signed-off-by: Eric Biggers <ebiggers@google.com>
---

v2: compare with previous value rather than first one.

 drivers/char/random.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index bf89c6f27a192..18d2d1f959683 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1382,12 +1382,22 @@ static void try_to_generate_entropy(void)
 		unsigned long entropy;
 		struct timer_list timer;
 	} stack;
+	int i;
 
+	/*
+	 * We must not proceed if we don't actually have a cycle counter.  To
+	 * detect a cycle counter, check whether random_get_entropy() returns a
+	 * new value each time.  Check this multiple times to avoid false
+	 * positives where a slow counter could be just on the cusp of a change.
+	 */
 	stack.entropy = random_get_entropy();
+	for (i = 0; i < 3; i++) {
+		unsigned long entropy = random_get_entropy();
 
-	/* Slow counter - or none. Don't even bother */
-	if (stack.entropy == random_get_entropy())
-		return;
+		if (stack.entropy == entropy)
+			return;
+		stack.entropy = entropy;
+	}
 
 	timer_setup_on_stack(&stack.timer, entropy_timer, 0);
 	while (!crng_ready() && !signal_pending(current)) {

base-commit: 939ee380b17589d026e132a1be91199409c3c934
-- 
2.35.2