linux-kernel - Re: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow protection

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Message-ID: <20170509015829.iycvxxbyblfgklsg@treble>
Date:   Mon, 8 May 2017 20:58:29 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        PaX Team <pageexec@...email.hu>, Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        David Windsor <dwindsor@...il.com>,
        "x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v3 2/2] x86/refcount: Implement fast refcount overflow
 protection

On Mon, May 08, 2017 at 04:31:11PM -0700, Kees Cook wrote:
> On Mon, May 8, 2017 at 3:53 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Mon, May 08, 2017 at 12:32:52PM -0700, Kees Cook wrote:
> >> +#define REFCOUNT_EXCEPTION                           \
> >> +     "movl $0x7fffffff, %[counter]\n\t"              \
> >> +     "int $"__stringify(X86_REFCOUNT_VECTOR)"\n"     \
> >> +     "0:\n\t"                                        \
> >> +     _ASM_EXTABLE(0b, 0b)
> >
> > Despite the objtool warnings going away, this still uses the exception
> > table in a new way, which will confuse objtool.  I need to do some more
> > thinking about the best way to fix it, either as a change to your patch
> > or a change to objtool.
> 
> In that it's not a "true" exception?

Right.  And also that it doesn't need the "fixup" since it would return
to the same address anyway.

-- 
Josh