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

[Woodhouse, David]

On Thu, 2014-08-21 at 06:19 +0930, Rusty Russell wrote:
> Arjun Sreedharan <arjun024@xxxxxxxxx> 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@xxxxxxxxx>
> 
> ...
> 
> >  	}
> > -	/* 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?

Yes, I think it's wrong.

I think I was under the impression that we shouldn't be freeing the old
'attrs' pointer on failure because it would live on in
{new,mk->mp}->grp.attrs and still be used. (Which I concede isn't
actually what the comment says, making it doubly wrong.)

I may have failed to notice that when this krealloc() fails, we actually
*free* the 'new' pointer and set mk->mp to NULL. Without freeing the
new->grp.attrs pointer. So it is indeed leaked. We *don't* need a
temporary variable for it though; we can find it.

In fact, I wonder if it's better to change the 'goto fail_free_new' in
the failure path here to instead do something like 'mk->mp = new; return
-ENOMEM;'. That way, the existing params would still be present and we'd
just have failed to add the *new* one. (And the leak wouldn't exist
because the 'attrs' pointer would live on as I originally thought.)

 
-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@xxxxxxxxx                              Intel Corporation

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