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

Eric Biggers
2022-04-21T23:32:48Z

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