linux-kernel - [PATCH 2/2] random: don't let 644 read-only sysctls be written to

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Date:   Mon, 28 Feb 2022 14:37:43 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Theodore Ts'o <tytso@....edu>
Subject: [PATCH 2/2] random: don't let 644 read-only sysctls be written to

We leave around these old sysctls for compatibility, and we keep them
"writable" for compatibility, but even after writing, we should keep
reporting the same value. This is consistent with how userspaces tend to
use sysctl_random_write_wakeup_bits, writing to it, and then later
reading from it and using the value.

Cc: Dominik Brodowski <linux@...inikbrodowski.net>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 drivers/char/random.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 116ebf50d791..06c6e15b5f3d 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1669,6 +1669,13 @@ static int proc_do_uuid(struct ctl_table *table, int write, void *buffer,
 	return proc_dostring(&fake_table, 0, buffer, lenp, ppos);
 }
 
+/* The same as proc_dointvec, but writes don't change anything. */
+static int proc_do_rointvec(struct ctl_table *table, int write, void *buffer,
+			    size_t *lenp, loff_t *ppos)
+{
+	return write ? 0 : proc_dointvec(table, write, buffer, lenp, ppos);
+}
+
 static struct ctl_table random_table[] = {
 	{
 		.procname	= "poolsize",
@@ -1689,14 +1696,14 @@ static struct ctl_table random_table[] = {
 		.data		= &sysctl_random_write_wakeup_bits,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_do_rointvec,
 	},
 	{
 		.procname	= "urandom_min_reseed_secs",
 		.data		= &sysctl_random_min_urandom_seed,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_do_rointvec,
 	},
 	{
 		.procname	= "boot_id",
-- 
2.35.1