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Date: Mon, 17 Feb 2020 10:23:55 -0800
From: Kees Cook <keescook@...omium.org>
To: zerons <zeronsaxm@...il.com>
Cc: Alexander Popov <alex.popov@...ux.com>,
Andrey Konovalov <andreyknvl@...gle.com>,
kernel-hardening@...ts.openwall.com, Shawn <citypw@...il.com>,
spender@...ecurity.net
Subject: Re: Maybe inappropriate use BUG_ON() in CONFIG_SLAB_FREELIST_HARDENED
On Mon, Feb 17, 2020 at 04:15:44PM +0100, Andrey Konovalov wrote:
> On Thu, Feb 13, 2020 at 4:43 PM zerons <zeronsaxm@...il.com> wrote:
> >
> > Hi,
> >
> > In slub.c(https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/mm/slub.c?h=v5.4.19#n305),
> > for SLAB_FREELIST_HARDENED, an extra detection of the double free bug has been added.
> >
> > This patch can (maybe only) detect something like this: kfree(a) kfree(a).
> > However, it does nothing when another process calls kfree(b) between the two kfree above.
> >
> > The problem is, if the panic_on_oops option is not set(Ubuntu 16.04/18.04 default option),
> > for a bug which kfree an object twice in a row, if another process can preempt the process
> > triggered this bug and then call kmalloc() to get the object, the patch doesn't work.
> >
> > Case 0: failure race
> > Process A:
> > kfree(a)
> > kfree(a)
> > the patch could terminate Process A.
> >
> > Case 1: race done
> > Process A:
> > kfree(a)
> > Process B:
> > kmalloc() -> a
> > Process A:
> > kfree(a)
> > the patch does nothing.
> >
> > The attacker can check the return status of process A to see if the race is done.
> >
> > Without this extra detection, the kernel could be unstable while the attacker
> > trying to do the race.
The check was just for the trivial case. It was an inexpensive check,
but was never designed to be a robust double-free defense.
> > In my opinion, this patch can somehow help attacker exploit this kind of bugs
> > more reliable.
Why do you think this makes races easier to win?
> +Alexander Popov, who is the author of the double free check in
> SLAB_FREELIST_HARDENED.
>
> Ah, so as long as the double free happens in a user process context,
> you can retry triggering it until you succeed in winning the race to
> reallocate the object (without causing slab freelist corruption, as it
> would have had happened before SLAB_FREELIST_HARDENED). Nice idea!
Do you see improvements that could be made here?
--
Kees Cook
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