[PATCH 5.17 020/111] random: remove outdated INT_MAX >> 6 check in urandom_read()

Greg Kroah-Hartman
2022-05-27T09:04:31Z

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@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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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;