From: Eric Biggers <ebiggers@xxxxxxxxxx> 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@xxxxxxxxxx> --- drivers/char/random.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/drivers/char/random.c b/drivers/char/random.c index bf89c6f27a192..9647c61345573 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1382,12 +1382,20 @@ static void try_to_generate_entropy(void) unsigned long entropy; struct timer_list timer; } stack; + int i; stack.entropy = random_get_entropy(); - /* Slow counter - or none. Don't even bother */ - if (stack.entropy == random_get_entropy()) - return; + /* + * 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 + * different value each time. Check it multiple times to avoid false + * positives where a slow counter could be just on the cusp of a change. + */ + for (i = 0; i < 3; i++) { + if (stack.entropy == random_get_entropy()) + return; + } timer_setup_on_stack(&stack.timer, entropy_timer, 0); while (!crng_ready() && !signal_pending(current)) { base-commit: 939ee380b17589d026e132a1be91199409c3c934 -- 2.35.2