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

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Message-ID: <CAGXu5j+zeCU083g6fDEcXM-CYEHYfbkd1cbWoUTJP_0WtyYRpA@mail.gmail.com>
Date:   Wed, 19 Jul 2017 15:50:14 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 12:52 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 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.

Ah! Now I see it. Gotcha.

>> >  })
>> > +
>> > +#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.

Right, yes.

BTW, I think this needs compiler.h coverage instead of the #else in
compiler-gcc.h (since it's different from how annotate_unreachable is
used only in compiler-gcc.h. I'll adjust.

Also, in looking at CONFIG_STACK_VALIDATION, do you want it to just
warn and skip, or do you want to error out the build if validation
isn't available but it's in the .config?

-Kees

-- 
Kees Cook
Pixel Security