linux-kernel - Re: [PATCH v2] ppdev: fix double-free of pp->pdev->name

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Date:   Thu, 10 Nov 2016 13:50:00 +0000
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Jann Horn <jann@...jh.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, linux-parport@...ts.infradead.org,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2] ppdev: fix double-free of pp->pdev->name

On Thu, Nov 10, 2016 at 02:18:12PM +0100, Arnd Bergmann wrote:
> On Sunday, October 30, 2016 11:19:24 PM CET Jann Horn wrote:
> > diff --git a/drivers/char/ppdev.c b/drivers/char/ppdev.c
> > index d23368874710..6af1ce04b3da 100644
> > --- a/drivers/char/ppdev.c
> > +++ b/drivers/char/ppdev.c
> > @@ -748,10 +748,7 @@ static int pp_release(struct inode *inode, struct file *file)
> >         }
> >  
> >         if (pp->pdev) {
> > -               const char *name = pp->pdev->name;
> > -
> >                 parport_unregister_device(pp->pdev);
> > -               kfree(name);
> >                 pp->pdev = NULL;
> >                 pr_debug(CHRDEV "%x: unregistered pardevice\n", minor);
> >         }
> > 
> 
> I took a closer look at this and found that we also leak the name
> that is passed in register_device() in the same file:
> 
>         name = kasprintf(GFP_KERNEL, CHRDEV "%x", minor);
> ...
>         pdev = parport_register_dev_model(port, name, &ppdev_cb, minor);
> 
> parport_register_dev_model() copies the name using kstrdup() and
> we should really free it after parport_register_dev_model().

yes. I missed that while converting the driver to use device model.
Thanks. I will send a patch to fix this tonight.

> 
> It's not a huge problem, just leaking a few bytes of memory, but
> the extra kfree() probably came from this confusion.

No, it was the old code which was not in the device-model. The old code
uses parport_register_device() which just uses the same string that is
passed to it and so it was freed while releasing the device.

Regards
Sudip