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

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Message-ID: <581633D3.3080502@gmail.com>
Date:   Sun, 30 Oct 2016 23:24:27 +0530
From:   Sudip Mukherjee <sudipm.mukherjee@...il.com>
To:     Jann Horn <jann@...jh.net>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     linux-kernel@...r.kernel.org, linux-parport@...ts.infradead.org,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH] ppdev: fix double-free of pp->pdev->name

On Sunday 30 October 2016 09:14 PM, Jann Horn wrote:
> free_pardevice() is called by parport_unregister_device() and already frees
> pp->pdev->name, don't try to do it again.
>
> This bug causes kernel crashes.
>
> I found and verified this with KASAN and some added pr_emerg()s:
>
> [   60.316568] pp_release: pp->pdev->name == ffff88039cb264c0
> [   60.316692] free_pardevice: freeing par_dev->name at ffff88039cb264c0
> [   60.316706] pp_release: kfree(ffff88039cb264c0)
> [   60.316714] ==========================================================
> [   60.316722] BUG: Double free or freeing an invalid pointer
> [   60.316731] Unexpected shadow byte: 0xFB
> [   60.316801] Object at ffff88039cb264c0, in cache kmalloc-32 size: 32
> [   60.316813] Allocated:
> [   60.316824] PID = 1695
> [   60.316869] Freed:
> [   60.316880] PID = 1695
> [   60.316935] ==========================================================
>
> CCing Andy Lutomirski because I think this is what broke vmapped stacks
> for me - after applying this patch, vmapped stacks worked for me.
> Previously, I got oopses (and lockups) caused by area->pages[0] being
> 0x400000000 in __vunmap(), with area->pages being allocated in the kmalloc
> area.

I think the above should not be a part of the commit message.
It looks like a valid bug. As we have now started using the device model 
from this release, so device_unregister() will finally call 
free_pardevice() and that will free the name.

>
> Signed-off-by: Jann Horn <jann@...jh.net>
> ---

Acked-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>

>   drivers/char/ppdev.c | 3 ---
>   1 file changed, 3 deletions(-)
>
> 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);
>   	}
>