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Message-ID: <20170112085714.GC3081@twins.programming.kicks-ass.net>
Date: Thu, 12 Jan 2017 09:57:14 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc: Kees Cook <keescook@...omium.org>,
"Reshetova, Elena" <elena.reshetova@...el.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"arnd@...db.de" <arnd@...db.de>,
"tglx@...utronix.de" <tglx@...utronix.de>,
"mingo@...hat.com" <mingo@...hat.com>,
"Anvin, H Peter" <h.peter.anvin@...el.com>,
"will.deacon@....com" <will.deacon@....com>,
"dwindsor@...il.com" <dwindsor@...il.com>,
"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
"ishkamiel@...il.com" <ishkamiel@...il.com>
Subject: Re: [RFC PATCH 08/19] kernel, mm: convert from
atomic_t to refcount_t
On Thu, Jan 12, 2017 at 02:11:15PM +0900, AKASHI Takahiro wrote:
> On Wed, Jan 11, 2017 at 02:55:21PM -0800, Kees Cook wrote:
> > On Wed, Jan 11, 2017 at 1:42 PM, Kees Cook <keescook@...omium.org> wrote:
> > > I can see if it'll cherry-pick cleanly, I assume it will. :)
> >
> > It cherry-picked cleanly. However, I made several changes:
> >
> > - I adjusted Peter's author email (it had extra []s around).
> > - I fixed all of the commit subjects (Peter's were missing).
> > - I added back "kref: Add KREF_INIT()" since it seems to have been
> > lost and mixed into other patches that would break bisection
> >
> > It's here now, please work from this version:
> >
> > http://git.kernel.org/cgit/linux/kernel/git/kees/linux.git/log/?h=kspp/hardened-atomic
>
> I gave it a spin on arm64.
> It can compile with a change to smp.c that I mentioned before,
> but the boot failed. I've not dug into it.
>
> ===8<===
> [ 3.578618] refcount_t: increment on 0; use-after-free.
> [ 3.579165] ------------[ cut here ]------------
> [ 3.579254] WARNING: CPU: 0 PID: 1 at /home/akashi/arm/armv8/linaro/linux-aarch64/include/linux/refcount.h:109 unx_create+0x8c/0xc0
That's dodgy code, someone needs to look at that.
It has an inc in a function called 'create' which seems to suggest its
objection creation and we should be using refcount_set() instead.
Then again, it looks like you can call this 'create' method multiple
times, each time returning the same static object, so refcount_set()
would not be correct.
Using a refcount on a static object is weird of course, so this is bound
to give trouble.
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