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

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Message-ID: <CALCETrXmvCuuH8YW=ttzOAMs0A10NKjDFPvDDm6hP7UhrXrUdg@mail.gmail.com>
Date:   Sun, 30 Oct 2016 09:29:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Jann Horn <jann@...jh.net>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-parport@...ts.infradead.org
Subject: Re: [PATCH] ppdev: fix double-free of pp->pdev->name

On Sun, Oct 30, 2016 at 8:44 AM, Jann Horn <jann@...jh.net> 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.

That's an odd symptom.  I assume that what's happening is that the
pages array is being freed early by the extra kfree in here and then
getting corrupted.

--Andy