linux-kernel - Re: [PATCH v3] kset: fix memory leak when kset

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Date:   Tue, 25 Oct 2022 12:51:00 -0400
From:   Luben Tuikov <luben.tuikov@....com>
To:     Yang Yingliang <yangyingliang@...wei.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-erofs@...ts.ozlabs.org, ocfs2-devel@....oracle.com,
        linux-mtd@...ts.infradead.org, amd-gfx@...ts.freedesktop.org
Cc:     gregkh@...uxfoundation.org, rafael@...nel.org, somlo@....edu,
        mst@...hat.com, jaegeuk@...nel.org, chao@...nel.org,
        hsiangkao@...ux.alibaba.com, huangjianan@...o.com, mark@...heh.com,
        jlbec@...lplan.org, joseph.qi@...ux.alibaba.com,
        akpm@...ux-foundation.org, alexander.deucher@....com,
        richard@....at, liushixin2@...wei.com
Subject: Re: [PATCH v3] kset: fix memory leak when kset_register() returns
 error

On 2022-10-25 03:15, Yang Yingliang wrote:
> Inject fault while loading module, kset_register() may fail.
> If it fails, the kset.kobj.name allocated by kobject_set_name()
> which must be called before a call to kset_register() may be
> leaked, since refcount of kobj was set in kset_init().

Technically, this is saying "If it fails, the kset.kobj.name may be leaked."
We want then to clarify that this is "allocated by kobj_set_name() which
must be called before a call to kset_register", so that needs to
be surrounded by commas (like a literary segue):

"If kset_register() fails, the kset.kobj.name allocated by kobject_set_name(),
 which must be called before a call to kset_register(), may be
 leaked."

I don't feel that the reason for the leak is "refcount of kobj was set in kset_init()".
It's a true statement, but not the reason for the leak--the reason for the leak is that
no one frees it on the error path.
 
> To mitigate this, we free the name in kset_register() when an
> error is encountered, i.e. when kset_register() returns an error.
> 
> A kset may be embedded in a larger structure which may be dynamically
> allocated in callers, it needs to be freed in ktype.release() or error

"_by_ callers", since it's something they _do_: allocate.

> path in callers, in this case, we can not call kset_put() in kset_register(),
> or it will cause double free, so just call kfree_const() to free the
> name and set it to NULL to avoid accessing bad pointer in callers.

That's good.

> With this fix, the callers don't need care about freeing the name
> and may call kset_put() if kset_register() fails.

"don't need to care about freeing the name" --> "don't need to free the name"

> Suggested-by: Luben Tuikov <luben.tuikov@....com>
> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> ---
> v2 -> v3:
>   Update commit message and comment of kset_register().
> 
> v1 -> v2:
>   Free name inside of kset_register() instead of calling kset_put()
>   in drivers.
> ---
>  lib/kobject.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/kobject.c b/lib/kobject.c
> index a0b2dbfcfa23..3cd19b9ca5ab 100644
> --- a/lib/kobject.c
> +++ b/lib/kobject.c
> @@ -834,6 +834,9 @@ EXPORT_SYMBOL_GPL(kobj_sysfs_ops);
>  /**
>   * kset_register() - Initialize and add a kset.
>   * @k: kset.
> + *
> + * NOTE: On error, the kset.kobj.name allocated by() kobj_set_name()
> + * is freed, it can not be used any more.

There's no need for "NOTE:"--it's just natural explanation of
what's happening, and what's expected in the doc comment to a function.
Drop it.

"is freed" is correct--that's a good change.

With these fixed, this patch is
Reviewed-by: <luben.tuikov@....com>

Regards,
Luben

>   */
>  int kset_register(struct kset *k)
>  {
> @@ -844,8 +847,12 @@ int kset_register(struct kset *k)
>  
>  	kset_init(k);
>  	err = kobject_add_internal(&k->kobj);
> -	if (err)
> +	if (err) {
> +		kfree_const(k->kobj.name);
> +		/* Set it to NULL to avoid accessing bad pointer in callers. */
> +		k->kobj.name = NULL;
>  		return err;
> +	}
>  	kobject_uevent(&k->kobj, KOBJ_ADD);
>  	return 0;
>  }