sysctl: fix proc_dobool() usability

Currently proc_dobool expects a (bool *) in table->data, but sizeof(int)
in table->maxsize, because it uses do_proc_dointvec() directly.

This is unsafe for at least two reasons:
1. A sysctl table definition may use { .data = &variable, .maxsize =
   sizeof(variable) }, not realizing that this makes the sysctl unusable
   (see the Fixes: tag) and that they need to use the completely
   counterintuitive sizeof(int) instead.
2. proc_dobool() will currently try to parse an array of values if given
   .maxsize >= 2*sizeof(int), but will try to write values of type bool
   by offsets of sizeof(int), so it will not work correctly with neither
   an (int *) nor a (bool *). There is no .maxsize validation to prevent
   this.

Fix this by:
1. Constraining proc_dobool() to allow only one value and .maxsize ==
   sizeof(bool).
2. Wrapping the original struct ctl_table in a temporary one with .data
   pointing to a local int variable and .maxsize set to sizeof(int) and
   passing this one to proc_dointvec(), converting the value to/from
   bool as needed (using proc_dou8vec_minmax() as an example).
3. Extending sysctl_check_table() to enforce proc_dobool() expectations.
4. Fixing the proc_dobool() docstring (it was just copy-pasted from
   proc_douintvec, apparently...).
5. Converting all existing proc_dobool() users to set .maxsize to
   sizeof(bool) instead of sizeof(int).

Fixes: 83efeeeb3d04 ("tty: Allow TIOCSTI to be disabled")
Fixes: a2071573d634 ("sysctl: introduce new proc handler proc_dobool")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

1 files changed, 24 insertions, 19 deletions

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 137d4abe3eda..1c240d2c99bc 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -425,21 +425,6 @@ 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)
@@ -710,16 +695,36 @@ 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 one integer value from/to the user buffer,
+ * treated as an ASCII string.
+ *
+ * table->data must point to a bool variable and table->maxlen must
+ * be sizeof(bool).
  *
  * 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;
+	bool *data = table->data;
+	int res, val;
+
+	/* Do not support arrays yet. */
+	if (table->maxlen != sizeof(bool))
+		return -EINVAL;
+
+	tmp = *table;
+	tmp.maxlen = sizeof(val);
+	tmp.data = &val;
+
+	val = READ_ONCE(*data);
+	res = proc_dointvec(&tmp, write, buffer, lenp, ppos);
+	if (res)
+		return res;
+	if (write)
+		WRITE_ONCE(*data, val);
+	return 0;
 }
 
 /**