linux-kernel - [PATCH] random: check for signal and try earlier when generating entropy

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Date:   Tue,  8 Mar 2022 10:18:49 -0700
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>
Subject: [PATCH] random: check for signal and try earlier when generating entropy

We call try_to_generate_entropy() from wait_for_random_bytes().
wait_for_random_bytes() always uses wait_event_interruptible_timeout()
when waiting, since it's called by userspace code in restartable
contexts, where signals can pend. When entering a busy loop in
try_to_generate_entropy(), we should therefore also check to see if any
signals are pending, so that a process doesn't get stuck in that loop
longer than expected. As well, there's no point in waiting for a full
second before trying to generate entropy; instead do it in the opposite
order, where we try to generate, and then go into the waitable.

Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index edb5b06544da..4c5f515b6080 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -128,11 +128,12 @@ int wait_for_random_bytes(void)

 	do {
 		int ret;
+
+		try_to_generate_entropy();
 		ret = wait_event_interruptible_timeout(crng_init_wait, crng_ready(), HZ);
 		if (ret)
 			return ret > 0 ? 0 : ret;

-		try_to_generate_entropy();
 	} while (!crng_ready());

 	return 0;
@@ -1374,7 +1375,7 @@ static void try_to_generate_entropy(void)
 		return;

 	timer_setup_on_stack(&stack.timer, entropy_timer, 0);
-	while (!crng_ready()) {
+	while (!crng_ready() && !signal_pending(current)) {
 		if (!timer_pending(&stack.timer))
 			mod_timer(&stack.timer, jiffies + 1);
 		mix_pool_bytes(&stack.cycles, sizeof(stack.cycles));
-- 
2.35.1