linux-kernel - [PATCH v2 9/9] random: remove outdated INT_MAX >> 6 check in urandom

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Date:   Wed,  9 Feb 2022 02:19:19 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     tytso@....edu, linux@...inikbrodowski.net, ebiggers@...nel.org,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH v2 9/9] random: remove outdated INT_MAX >> 6 check in urandom_read()

In 79a8468747c5 ("random: check for increase of entropy_count because of
signed conversion"), a number of checks were added around what values
were passed to account(), because account() was doing fancy fixed point
fractional arithmetic, and a user had some ability to pass large values
directly into it. One of things in that commit was limiting those values
to INT_MAX >> 6.

However, for several years now, urandom reads no longer touch entropy
accounting, and so this check serves no purpose. The current flow is:

urandom_read_nowarn()-->get_random_bytes_user()-->chacha20_block()

We arrive at urandom_read_nowarn() in the first place either via
ordinary fops, which limits reads to MAX_RW_COUNT, or via getrandom()
which limits reads to INT_MAX.

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

diff --git a/drivers/char/random.c b/drivers/char/random.c
index f7f9cbfe13f7..e09874c511d0 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1305,7 +1305,6 @@ static ssize_t urandom_read_nowarn(struct file *file, char __user *buf,
 {
 	int ret;

-	nbytes = min_t(size_t, nbytes, INT_MAX >> 6);
 	ret = get_random_bytes_user(buf, nbytes);
 	trace_urandom_read(8 * nbytes, 0, input_pool.entropy_count);
 	return ret;
-- 
2.35.0