Add the __counted_by compiler attribute to the flexible array member
attrs to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Increment num before adding a new param_attribute to the attrs array and
adjust the array index accordingly. Increment num immediately after the
first reallocation such that krealloc() for the NULL terminator only
needs to add 1 (instead of 2) to mk->mp->num.

Use struct_size() instead of manually calculating the size for the
reallocation.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
---
 kernel/params.c | 26 ++++++++++++--------------
 1 file changed, 12 insertions(+), 14 deletions(-)

diff --git a/kernel/params.c b/kernel/params.c
index 2e447f8ae183..160b66dbc0b0 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -551,7 +551,7 @@ struct module_param_attrs
 {
 	unsigned int num;
 	struct attribute_group grp;
-	struct param_attribute attrs[];
+	struct param_attribute attrs[] __counted_by(num);
 };
 
 #ifdef CONFIG_SYSFS
@@ -651,35 +651,33 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
 	}
 
 	/* Enlarge allocations. */
-	new_mp = krealloc(mk->mp,
-			  sizeof(*mk->mp) +
-			  sizeof(mk->mp->attrs[0]) * (mk->mp->num + 1),
+	new_mp = krealloc(mk->mp, struct_size(mk->mp, attrs, mk->mp->num + 1),
 			  GFP_KERNEL);
 	if (!new_mp)
 		return -ENOMEM;
 	mk->mp = new_mp;
+	mk->mp->num++;
 
 	/* Extra pointer for NULL terminator */
 	new_attrs = krealloc(mk->mp->grp.attrs,
-			     sizeof(mk->mp->grp.attrs[0]) * (mk->mp->num + 2),
+			     sizeof(mk->mp->grp.attrs[0]) * (mk->mp->num + 1),
 			     GFP_KERNEL);
 	if (!new_attrs)
 		return -ENOMEM;
 	mk->mp->grp.attrs = new_attrs;
 
 	/* Tack new one on the end. */
-	memset(&mk->mp->attrs[mk->mp->num], 0, sizeof(mk->mp->attrs[0]));
-	sysfs_attr_init(&mk->mp->attrs[mk->mp->num].mattr.attr);
-	mk->mp->attrs[mk->mp->num].param = kp;
-	mk->mp->attrs[mk->mp->num].mattr.show = param_attr_show;
+	memset(&mk->mp->attrs[mk->mp->num - 1], 0, sizeof(mk->mp->attrs[0]));
+	sysfs_attr_init(&mk->mp->attrs[mk->mp->num - 1].mattr.attr);
+	mk->mp->attrs[mk->mp->num - 1].param = kp;
+	mk->mp->attrs[mk->mp->num - 1].mattr.show = param_attr_show;
 	/* Do not allow runtime DAC changes to make param writable. */
 	if ((kp->perm & (S_IWUSR | S_IWGRP | S_IWOTH)) != 0)
-		mk->mp->attrs[mk->mp->num].mattr.store = param_attr_store;
+		mk->mp->attrs[mk->mp->num - 1].mattr.store = param_attr_store;
 	else
-		mk->mp->attrs[mk->mp->num].mattr.store = NULL;
-	mk->mp->attrs[mk->mp->num].mattr.attr.name = (char *)name;
-	mk->mp->attrs[mk->mp->num].mattr.attr.mode = kp->perm;
-	mk->mp->num++;
+		mk->mp->attrs[mk->mp->num - 1].mattr.store = NULL;
+	mk->mp->attrs[mk->mp->num - 1].mattr.attr.name = (char *)name;
+	mk->mp->attrs[mk->mp->num - 1].mattr.attr.mode = kp->perm;
 
 	/* Fix up all the pointers, since krealloc can move us */
 	for (i = 0; i < mk->mp->num; i++)
-- 
2.46.0

On Fri, Aug 23, 2024 at 02:33:00PM +0200, Thorsten Blum wrote:
> Add the __counted_by compiler attribute to the flexible array member
> attrs to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
> CONFIG_FORTIFY_SOURCE.
> 
> Increment num before adding a new param_attribute to the attrs array and
> adjust the array index accordingly. Increment num immediately after the
> first reallocation such that krealloc() for the NULL terminator only
> needs to add 1 (instead of 2) to mk->mp->num.
> 
> Use struct_size() instead of manually calculating the size for the
> reallocation.

...

>  	/* Extra pointer for NULL terminator */
>  	new_attrs = krealloc(mk->mp->grp.attrs,
> -			     sizeof(mk->mp->grp.attrs[0]) * (mk->mp->num + 2),
> +			     sizeof(mk->mp->grp.attrs[0]) * (mk->mp->num + 1),
>  			     GFP_KERNEL);

Convert this to use krealloc_array().

>  	if (!new_attrs)
>  		return -ENOMEM;


-- 
With Best Regards,
Andy Shevchenko