[PATCH v3] sysctl: handle table->maxlen robustly for proc_dobool

[Muchun Song <songmuchun@bytedance.com>]

Setting ->proc_handler to proc_dobool at the same time setting ->maxlen
to sizeof(int) is counter-intuitive, it is easy to make mistakes in the
future (When I first use proc_dobool() in my driver, I assign
sizeof(variable) to table->maxlen.  Then I found it was wrong, it should
be sizeof(int) which was very counter-intuitive).  For robustness,
rework proc_dobool() robustly.  So it is an improvement not a real bug
fix.

Signed-off-by: Muchun Song <songmuchun@bytedance.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Iurii Zaikin <yzaikin@google.com>
---
v3:
 - Update commit log.

v2:
 - Reimplementing proc_dobool().

 fs/lockd/svc.c  |  2 +-
 kernel/sysctl.c | 38 +++++++++++++++++++-------------------
 2 files changed, 20 insertions(+), 20 deletions(-)

diff --git a/fs/lockd/svc.c b/fs/lockd/svc.c
index 59ef8a1f843f..6e48ee787f49 100644
--- a/fs/lockd/svc.c
+++ b/fs/lockd/svc.c
@@ -496,7 +496,7 @@ static struct ctl_table nlm_sysctls[] = {
 	{
 		.procname	= "nsm_use_hostnames",
 		.data		= &nsm_use_hostnames,
-		.maxlen		= sizeof(int),
+		.maxlen		= sizeof(nsm_use_hostnames),
 		.mode		= 0644,
 		.proc_handler	= proc_dobool,
 	},
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index e52b6e372c60..50a2c29efc94 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -423,21 +423,6 @@ static void proc_put_char(void **buf, size_t *size, char c)
 	}
 }
 
-static int do_proc_dobool_conv(bool *negp, unsigned long *lvalp,
-				int *valp,
-				int write, void *data)
-{
-	if (write) {
-		*(bool *)valp = *lvalp;
-	} else {
-		int val = *(bool *)valp;
-
-		*lvalp = (unsigned long)val;
-		*negp = false;
-	}
-	return 0;
-}
-
 static int do_proc_dointvec_conv(bool *negp, unsigned long *lvalp,
 				 int *valp,
 				 int write, void *data)
@@ -708,16 +693,31 @@ int do_proc_douintvec(struct ctl_table *table, int write,
  * @lenp: the size of the user buffer
  * @ppos: file position
  *
- * Reads/writes up to table->maxlen/sizeof(unsigned int) integer
- * values from/to the user buffer, treated as an ASCII string.
+ * Reads/writes up to table->maxlen/sizeof(bool) bool values from/to
+ * the user buffer, treated as an ASCII string.
  *
  * Returns 0 on success.
  */
 int proc_dobool(struct ctl_table *table, int write, void *buffer,
 		size_t *lenp, loff_t *ppos)
 {
-	return do_proc_dointvec(table, write, buffer, lenp, ppos,
-				do_proc_dobool_conv, NULL);
+	struct ctl_table tmp = *table;
+	bool *data = table->data;
+	unsigned int val = READ_ONCE(*data);
+	int ret;
+
+	/* Do not support arrays yet. */
+	if (table->maxlen != sizeof(bool))
+		return -EINVAL;
+
+	tmp.maxlen = sizeof(val);
+	tmp.data = &val;
+	ret = do_proc_douintvec(&tmp, write, buffer, lenp, ppos, NULL, NULL);
+	if (ret)
+		return ret;
+	if (write)
+		WRITE_ONCE(*data, val ? true : false);
+	return 0;
 }
 
 /**
-- 
2.11.0