random: remove outdated INT_MAX >> 6 check in urandom_read()

stable inclusion
from stable-v5.10.119
commit 63c1aae40ac16ed136188bb9c9b595c0147fe6fa
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5L6BB

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63c1aae40ac16ed136188bb9c9b595c0147fe6fa

--------------------------------

commit 434537ae upstream.

In 79a84687 ("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: NDominik Brodowski <linux@dominikbrodowski.net>
Reviewed-by: NJann Horn <jannh@google.com>
Reviewed-by: NEric Biggers <ebiggers@google.com>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>
Acked-by: NXie XiuQi <xiexiuqi@huawei.com>

drivers/char/random.c

@@ -1286,9 +1286,8 @@ void rand_initialize_disk(struct gendisk *disk)
 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;