linux-kernel - Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow protection

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Message-ID: <20170719195249.akr6m2x65mhtsyvf@treble>
Date:   Wed, 19 Jul 2017 14:52:49 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Christoph Hellwig <hch@...radead.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        Eric Biggers <ebiggers3@...il.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Hans Liljestrand <ishkamiel@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Serge E. Hallyn" <serge@...lyn.com>, arozansk@...hat.com,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        "x86@...nel.org" <x86@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        "David S. Miller" <davem@...emloft.net>,
        Rik van Riel <riel@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v6 2/2] x86/refcount: Implement fast refcount overflow
 protection

On Wed, Jul 19, 2017 at 12:45:19PM -0700, Kees Cook wrote:
> > diff --git a/arch/x86/include/asm/refcount.h b/arch/x86/include/asm/refcount.h
> > index 13b91e850a02..e7587db3487c 100644
> > --- a/arch/x86/include/asm/refcount.h
> > +++ b/arch/x86/include/asm/refcount.h
> > @@ -15,6 +15,7 @@
> >         ".pushsection .text.unlikely\n"                 \
> >         "111:\tlea %[counter], %%" _ASM_CX "\n"         \
> >         "112:\t" ASM_UD0 "\n"                           \
> > +       ASM_UNREACHABLE                                 \
> >         ".popsection\n"                                 \
> >         "113:\n"                                        \
> >         _ASM_EXTABLE_REFCOUNT(112b, 113b)
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index cd4bbe8242bd..85e0b8f42ca0 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -202,15 +202,25 @@
> >  #endif
> >
> >  #ifdef CONFIG_STACK_VALIDATION
> > +
> >  #define annotate_unreachable() ({                                      \
> >         asm("%c0:\t\n"                                                  \
> > -           ".pushsection .discard.unreachable\t\n"                     \
> > -           ".long %c0b - .\t\n"                                        \
> > -           ".popsection\t\n" : : "i" (__LINE__));                      \
> > +           ".pushsection .discard.unreachable\n\t"                     \
> > +           ".long %c0b - .\n\t"                                        \
> > +           ".popsection\n\t" : : "i" (__LINE__));                      \
> 
> Is this just an indentation change?

This was sneaking in a fix to put the tab after the newline instead of
before it.  I figured it's not worth its own commit.

> >  })
> > +
> > +#define ASM_UNREACHABLE                                                        \
> > +       "999: .pushsection .discard.unreachable\n\t"                    \
> > +       ".long 999b - .\n\t"                                            \
> > +       ".popsection\n\t"
> 
> Just so I understand, we'll get a single byte added for each exception
> case, but it'll get discarded during final link?

I think it's four bytes actually, but yeah, the section gets stripped at
vmlinux link time.

-- 
Josh