linux-kernel - Re: [PATCH] params: fix potential memory leak in add_sysfs

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Message-ID: <87ha16izpu.fsf@rustcorp.com.au>
Date:	Thu, 21 Aug 2014 07:35:33 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Arjun Sreedharan <arjun024@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jingoo Han <jg1.han@...sung.com>, linux-kernel@...r.kernel.org,
	David Woodhouse <David.Woodhouse@...el.com>
Subject: Re: [PATCH] params: fix potential memory leak in add_sysfs_param()

Arjun Sreedharan <arjun024@...il.com> writes:
> On 21 August 2014 02:19, Rusty Russell <rusty@...tcorp.com.au> wrote:
>> Arjun Sreedharan <arjun024@...il.com> writes:
>>> Do not leak memory when attrs is non NULL and
>>> krealloc() fails. Without temporary variable,
>>> reference to it is lost.
>>>
>>> Signed-off-by: Arjun Sreedharan <arjun024@...il.com>
>>
>> ...
>>
>>>       }
>>> -     /* 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. */
>>> -     attrs = krealloc(attrs, sizeof(new->grp.attrs[0])*(num+2), GFP_KERNEL);
>>> -     if (!attrs) {
>>> +
>>> +     new_attrs = krealloc(attrs, sizeof(new->grp.attrs[0])*(num+2), GFP_KERNEL);
>>> +     if (!new_attrs) {
>>
>> I think that comment you deleted is pretty clear.  Is it wrong?
>>
>> Cheers,
>> Rusty.
>
> I believe it's wrong. I do not understand how `this is safe` from memory leak.
> If krealloc() fails, there is nothing in place to free memory held by @attrs

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.

But David was the one who placed this comment; I'll let him figure out
whether it's bogus or not :)

Cheers,
Rusty.

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