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

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Date:   Thu, 10 Nov 2016 14:18:12 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Jann Horn <jann@...jh.net>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        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 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().

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

	Arnd