linux-kernel - [PATCH 5.15 055/145] random: remove outdated INT_MAX >> 6 check in urandom

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Message-Id: <20220527084857.418041293@linuxfoundation.org>
Date:   Fri, 27 May 2022 10:49:16 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Theodore Tso <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH 5.15 055/145] random: remove outdated INT_MAX >> 6 check in urandom_read()

From: "Jason A. Donenfeld" <Jason@...c4.com>

commit 434537ae54ad37e93555de21b6ac8133d6d773a9 upstream.

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. The first >> 3 was for bytes to bits, and the next >> 3
was for bits to 1/8 fractional bits.

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()

Of course, we don't want that size_t to be truncated when adding it into
the ssize_t. But 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>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Reviewed-by: Jann Horn <jannh@...gle.com>
Reviewed-by: Eric Biggers <ebiggers@...gle.com>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/char/random.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1286,9 +1286,8 @@ void rand_initialize_disk(struct gendisk
 static ssize_t urandom_read_nowarn(struct file *file, char __user *buf,
 				   size_t nbytes, loff_t *ppos)
 {
-	int ret;
+	ssize_t ret;

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