linux-kernel - Re: [PATCH] locking/refcounts, x86/asm: Use unique .text section for refcount exceptions

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Date:   Sat, 02 Sep 2017 06:03:11 +0200
From:   Mike Galbraith <efault@....de>
To:     Kees Cook <keescook@...omium.org>, Ingo Molnar <mingo@...e.hu>
Cc:     x86@...nel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: [PATCH] locking/refcounts, x86/asm: Use unique .text section
 for refcount exceptions

On Fri, 2017-09-01 at 13:22 -0700, Kees Cook wrote:
> Using .text.unlikely for refcount exceptions isn't safe because gcc may
> move entire functions into .text.unlikely (e.g. in6_dev_get()), which
> would cause any uses of a protected refcount_t function to stay inline
> with the function, triggering the protection unconditionally:
> 
>         .section        .text.unlikely,"ax",@progbits
>         .type   in6_dev_get, @function
> in6_dev_getx:
> .LFB4673:
>         .loc 2 4128 0
>         .cfi_startproc
> ...
>         lock; incl 480(%rbx)
>         js 111f
>         .pushsection .text.unlikely
> 111:    lea 480(%rbx), %rcx
> 112:    .byte 0x0f, 0xff
> .popsection
> 113:
> 
> This creates a unique .text section and adds an additional test to the
> exception handler to WARN in the case of having none of OF, SF, nor ZF
> set so we can see things like this more easily in the future.

Closure: gcc-4.8.5 now builds a functional kernel as well, so that
aspect of this bug was just a larger a dose of the same toxin.

Question below.

diff --git a/arch/x86/include/asm/refcount.h
b/arch/x86/include/asm/refcount.h
> index ff871210b9f2..4e44250e7d0d 100644
> --- a/arch/x86/include/asm/refcount.h
> +++ b/arch/x86/include/asm/refcount.h
> @@ -15,7 +15,7 @@
>   * back to the regular execution flow in .text.
>   */
>  #define _REFCOUNT_EXCEPTION				\
> -	".pushsection .text.unlikely\n"			\
> +	".pushsection .text..refcount\n"		\

Why two dots? (.text.refcount_ex?)

	-Mike