linux-kernel - [PATCH 4.9 037/159] random: restore O

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Date:   Mon, 24 Oct 2022 13:29:51 +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, Guozihua <guozihua@...wei.com>,
        Zhongguohua <zhongguohua1@...wei.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Theodore Tso <tytso@....edu>,
        Andrew Lutomirski <luto@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: [PATCH 4.9 037/159] random: restore O_NONBLOCK support

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

commit cd4f24ae9404fd31fc461066e57889be3b68641b upstream.

Prior to 5.6, when /dev/random was opened with O_NONBLOCK, it would
return -EAGAIN if there was no entropy. When the pools were unified in
5.6, this was lost. The post 5.6 behavior of blocking until the pool is
initialized, and ignoring O_NONBLOCK in the process, went unnoticed,
with no reports about the regression received for two and a half years.
However, eventually this indeed did break somebody's userspace.

So we restore the old behavior, by returning -EAGAIN if the pool is not
initialized. Unlike the old /dev/random, this can only occur during
early boot, after which it never blocks again.

In order to make this O_NONBLOCK behavior consistent with other
expectations, also respect users reading with preadv2(RWF_NOWAIT) and
similar.

Fixes: 30c08efec888 ("random: make /dev/random be almost like /dev/urandom")
Reported-by: Guozihua <guozihua@...wei.com>
Reported-by: Zhongguohua <zhongguohua1@...wei.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: Theodore Ts'o <tytso@....edu>
Cc: Andrew Lutomirski <luto@...nel.org>
Cc: stable@...r.kernel.org
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 drivers/char/random.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1295,6 +1295,10 @@ static ssize_t random_read_iter(struct k
 {
 	int ret;

+	if (!crng_ready() &&
+	    (kiocb->ki_filp->f_flags & O_NONBLOCK))
+		return -EAGAIN;
+
 	ret = wait_for_random_bytes();
 	if (ret != 0)
 		return ret;