From 48bff1053c172e6c7f340e506027d118147c8b7f Mon Sep 17 00:00:00 2001 From: "Jason A. Donenfeld" Date: Tue, 5 Apr 2022 15:57:05 +0200 Subject: [PATCH] random: opportunistically initialize on /dev/urandom reads In 6f98a4bfee72 ("random: block in /dev/urandom"), we tried to make a successful try_to_generate_entropy() call *required* if the RNG was not already initialized. Unfortunately, weird architectures and old userspaces combined in TCG test harnesses, making that change still not realistic, so it was reverted in 0313bc278dac ("Revert "random: block in /dev/urandom""). However, rather than making a successful try_to_generate_entropy() call *required*, we can instead make it *best-effort*. If try_to_generate_entropy() fails, it fails, and nothing changes from the current behavior. If it succeeds, then /dev/urandom becomes safe to use for free. This way, we don't risk the regression potential that led to us reverting the required-try_to_generate_entropy() call before. Practically speaking, this means that at least on x86, /dev/urandom becomes safe. Probably other architectures with working cycle counters will also become safe. And architectures with slow or broken cycle counters at least won't be affected at all by this change. So it may not be the glorious "all things are unified!" change we were hoping for initially, but practically speaking, it makes a positive impact. Cc: Theodore Ts'o Cc: Dominik Brodowski Cc: Linus Torvalds Signed-off-by: Jason A. Donenfeld --- drivers/char/random.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/drivers/char/random.c b/drivers/char/random.c index ee3ad2ba0942..388025d6d38d 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -1534,6 +1534,13 @@ static ssize_t urandom_read(struct file *file, char __user *buf, size_t nbytes, { static int maxwarn = 10; + /* + * Opportunistically attempt to initialize the RNG on platforms that + * have fast cycle counters, but don't (for now) require it to succeed. + */ + if (!crng_ready()) + try_to_generate_entropy(); + if (!crng_ready() && maxwarn > 0) { maxwarn--; if (__ratelimit(&urandom_warning))