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: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -1631,6 +1631,13 @@ 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, 0, buffer, lenp, ppos);
+}
+
 static struct ctl_table random_table[] = {
 	{
 		.procname	= "poolsize",
@@ -1651,14 +1658,14 @@ 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",