Re: [PATCH] params: fix potential memory leak in add_sysfs_param()

Woodhouse, David
2014-08-20T22:17:54Z

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On Thu, 2014-08-21 at 07:35 +0930, Rusty Russell wrote:
>
> Above this:
> if (!mk->mp) {
> num = 0;
> attrs = NULL;
> } else {
> num = mk->mp->num;
> attrs = mk->mp->grp.attrs;
> }
>
> So, attrs is just a temporary: either NULL (doesn't need freeing), or
> is the old mk->mp->grp.attrs ptr.

Except that in the failure case we *free* the old mk->mp and never free
mk->mp->grp.attrs so it *is* indeed lost.

A simpler version of Arjun's patch might look like this:

diff --git a/kernel/params.c b/kernel/params.c
index 34f5270..f9459bc 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -613,7 +613,6 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
sizeof(*mk->mp) + sizeof(mk->mp->attrs[0]) * (num+1),
GFP_KERNEL);
if (!new) {
- kfree(attrs);
err = -ENOMEM;
goto fail;
}
@@ -653,7 +652,10 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
fail_free_new:
kfree(new);
fail:
- mk->mp = NULL;
+ if (mk->mp) {
+ kfree(mk->mp->grp.attrs);
+ mk->mp = NULL;
+ }
return err;
}

But as I suggested in my previous response, a *better* fix might look
like this:

diff --git a/kernel/params.c b/kernel/params.c
index 34f5270..cdab9d4 100644
--- a/kernel/params.c
+++ b/kernel/params.c
@@ -595,7 +595,7 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
{
struct module_param_attrs *new;
struct attribute **attrs;
- int err, num;
+ int num;

/* We don't bother calling this with invisible parameters. */
BUG_ON(!kp->perm);
@@ -612,18 +612,19 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,
new = krealloc(mk->mp,
sizeof(*mk->mp) + sizeof(mk->mp->attrs[0]) * (num+1),
GFP_KERNEL);
- if (!new) {
- kfree(attrs);
- err = -ENOMEM;
- goto fail;
- }
+ if (!new)
+ return -ENOMEM;
+
/* Despite looking like the typical realloc() bug, this is safe.
- * We *want* the old 'attrs' to be freed either way, and we'll store
- * the new one in the success case. */
+ * In the failure case, the old 'attrs' is still in new->grp.attrs
+ * and will live on there. */
attrs = krealloc(attrs, sizeof(new->grp.attrs[0])*(num+2), GFP_KERNEL);
if (!attrs) {
- err = -ENOMEM;
- goto fail_free_new;
+ /* This is in a larger kmalloc allocation than before but
+ * otherwise entirely unchanged. We've failed to add the
+ * new param but the existing ones are still there. */
+ mk->mp = new;
+ return -ENOMEM;
}

/* Sysfs wants everything zeroed. */
@@ -649,12 +650,6 @@ static __modinit int add_sysfs_param(struct module_kobject *mk,

mk->mp = new;
return 0;
-
-fail_free_new:
- kfree(new);
-fail:
- mk->mp = NULL;
- return err;
}

#ifdef CONFIG_MODULES

--
David Woodhouse Open Source Technology Centre
David.Woodhouse@intel.com Intel Corporation

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