[PATCH v2 2/3] params: Fix an overflow in param_attr_show

[Jean Delvare <jdelvare@suse.de>]

Function param_attr_show could overflow the buffer it is operating
on. The buffer size is PAGE_SIZE, and the string returned by
attribute->param->ops->get is generated by scnprintf(buffer,
PAGE_SIZE, ...) so it could be PAGE_SIZE - 1 long, with the
terminating '\0' at the very end of the buffer. Calling
strcat(..., "\n") on this isn't safe, as the '\0' will be replaced
by '\n' (OK) and then another '\0' will be added past the end of
the buffer (not OK.)

Simply add the trailing '\n' when writing the attribute contents to
the buffer originally. This is safe, and also faster.

Credits to Teradata for discovering this issue.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
---
Changes since v1:
* Add the \n inside the STANDARD_PARAM_DEF macro instead of changing
  all the callers.
* Handle boolean and string types too.
* Handle parameter arrays properly.

 kernel/params.c |   17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

--- linux-4.13.orig/kernel/params.c	2017-09-28 11:09:47.573434740 +0200
+++ linux-4.13/kernel/params.c	2017-09-28 14:40:16.625647445 +0200
@@ -224,7 +224,7 @@ char *parse_args(const char *doing,
 	}								\
 	int param_get_##name(char *buffer, const struct kernel_param *kp) \
 	{								\
-		return scnprintf(buffer, PAGE_SIZE, format,		\
+		return scnprintf(buffer, PAGE_SIZE, format "\n",	\
 				*((type *)kp->arg));			\
 	}								\
 	const struct kernel_param_ops param_ops_##name = {			\
@@ -270,7 +270,7 @@ EXPORT_SYMBOL(param_set_charp);
 
 int param_get_charp(char *buffer, const struct kernel_param *kp)
 {
-	return scnprintf(buffer, PAGE_SIZE, "%s", *((char **)kp->arg));
+	return scnprintf(buffer, PAGE_SIZE, "%s\n", *((char **)kp->arg));
 }
 EXPORT_SYMBOL(param_get_charp);
 
@@ -301,7 +301,7 @@ EXPORT_SYMBOL(param_set_bool);
 int param_get_bool(char *buffer, const struct kernel_param *kp)
 {
 	/* Y and N chosen as being relatively non-coder friendly */
-	return sprintf(buffer, "%c", *(bool *)kp->arg ? 'Y' : 'N');
+	return sprintf(buffer, "%c\n", *(bool *)kp->arg ? 'Y' : 'N');
 }
 EXPORT_SYMBOL(param_get_bool);
 
@@ -360,7 +360,7 @@ EXPORT_SYMBOL(param_set_invbool);
 
 int param_get_invbool(char *buffer, const struct kernel_param *kp)
 {
-	return sprintf(buffer, "%c", (*(bool *)kp->arg) ? 'N' : 'Y');
+	return sprintf(buffer, "%c\n", (*(bool *)kp->arg) ? 'N' : 'Y');
 }
 EXPORT_SYMBOL(param_get_invbool);
 
@@ -460,8 +460,9 @@ static int param_array_get(char *buffer,
 	struct kernel_param p = *kp;
 
 	for (i = off = 0; i < (arr->num ? *arr->num : arr->max); i++) {
+		/* Replace \n with comma */
 		if (i)
-			buffer[off++] = ',';
+			buffer[off - 1] = ',';
 		p.arg = arr->elem + arr->elemsize * i;
 		check_kparam_locked(p.mod);
 		ret = arr->ops->get(buffer + off, &p);
@@ -507,7 +508,7 @@ EXPORT_SYMBOL(param_set_copystring);
 int param_get_string(char *buffer, const struct kernel_param *kp)
 {
 	const struct kparam_string *kps = kp->str;
-	return strlcpy(buffer, kps->string, PAGE_SIZE);
+	return scnprintf(buffer, PAGE_SIZE, "%s\n", kps->string);
 }
 EXPORT_SYMBOL(param_get_string);
 
@@ -549,10 +550,6 @@ static ssize_t param_attr_show(struct mo
 	kernel_param_lock(mk->mod);
 	count = attribute->param->ops->get(buf, attribute->param);
 	kernel_param_unlock(mk->mod);
-	if (count > 0) {
-		strcat(buf, "\n");
-		++count;
-	}
 	return count;
 }
 

-- 
Jean Delvare
SUSE L3 Support