linux-kernel - Re: [PATCH v2] kset: fix memory leak when kset

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Date:   Mon, 24 Oct 2022 17:06:52 -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 v2] kset: fix memory leak when kset_register() returns
 error

On 2022-10-24 08:19, Yang Yingliang wrote:
> Inject fault while loading module, kset_register() may fail.
> If it fails, the name allocated by kobject_set_name() which
> is called before kset_register() is leaked, because refcount
> of kobject is hold in kset_init().

"is hold" --> "was set".

Also, I'd say "which must be called" instead of "is", since
we cannot register kobj/kset without a name--the kobj code crashes,
and we want to make this clear. IOW, a novice user may wonder
where "is" it called, as opposed to learning that they "must"
call it to allocate/set a name, before calling kset_register().

So, I'd say this:

"If it fails, the name allocated by kobject_set_name() which must
 be called before a call to kset_regsiter() is leaked, since
 refcount of kobj was set in kset_init()."

> 
> As a kset may be embedded in a larger structure which needs
> be freed in release() function or error path in callers, we

Drop "As", start with "A kset". "which needs _to_ be".
Also please specify that the release is part of the ktype,
like this:

"A kset may be embedded in a larger structure which needs to be
 freed in ktype.release() or error path in callers, 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.
> 
> With this fix, the callers don't need to care about the name
> freeing and call an extra kset_put() if kset_register() fails.

This is unclear because you're *missing* a verb:
"and call an extra kset_put()".
Please add the proper verb _between_ "and call", something like,

"With this fix, the callers don't need to care about freeing
 the name of the kset, and _can_ call kset_put() if kset_register() fails."

Choose a proper verb here: can, should, cannot, should not, etc.

We can do this because you set "kset.kobj.name to NULL, and this
is checked for in kobject_cleanup(). We just need to stipulate
whether they should/shouldn't have to call kset_put(), or can free the kset
and/or the embedding object themselves. This really depends
on how we want kset_register() to behave in the future, and on
user's own ktype.release implementation...

> 
> Suggested-by: Luben Tuikov <luben.tuikov@....com>
> Signed-off-by: Yang Yingliang <yangyingliang@...wei.com>
> ---
> v1 -> v2:
>   Free name inside of kset_register() instead of calling kset_put()
>   in drivers.
> ---
>  lib/kobject.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/kobject.c b/lib/kobject.c
> index a0b2dbfcfa23..3409a89c81e5 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()
> + * which is called before kset_register() in caller need be freed.
>   */
>  int kset_register(struct kset *k)
>  {
> @@ -844,8 +847,11 @@ int kset_register(struct kset *k)
>  
>  	kset_init(k);
>  	err = kobject_add_internal(&k->kobj);
> -	if (err)
> +	if (err) {
> +		kfree_const(k->kobj.name);
> +		k->kobj.name = NULL;
>  		return err;
> +	}

This looks good. It's good you set kset.kobj.name to NULL, so that
recovery/free paths don't get confused. Waiting for v3.

(I guess this is no different than what we currently do in kobject_cleanup(),
 so I see it as safe, no-surprises implementation.)

Regards,
Luben