From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Mon, 7 Feb 2022 23:37:13 +0100
Subject: random: remove outdated INT_MAX >> 6 check in urandom_read()
Patch-mainline: v5.18-rc1
Git-commit: 434537ae54ad37e93555de21b6ac8133d6d773a9
References: bsc#1204911

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@mit.edu>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: Jann Horn <jannh@google.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
[nstange@suse.de: adapted diff context for backport]
Acked-by: Nicolai Stange <nstange@suse.de>
---
 drivers/char/random.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1836,9 +1836,8 @@ 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 >> (ENTROPY_SHIFT + 3));
 	ret = extract_crng_user(buf, nbytes);
 	trace_urandom_read(8 * nbytes, 0, ENTROPY_BITS(&input_pool));
 	return ret;