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

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Message-ID: <20170508225308.a6uznrhdm7pgyhcg@treble>
Date:   Mon, 8 May 2017 17:53:08 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     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 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.

> diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
> index 532372c6cf15..0590f384f234 100644
> --- a/include/asm-generic/sections.h
> +++ b/include/asm-generic/sections.h
> @@ -20,6 +20,8 @@
>   *                   may be out of this range on some architectures.
>   * [_sinittext, _einittext]: contains .init.text.* sections
>   * [__bss_start, __bss_stop]: contains BSS sections
> + * [__refcount_overflow/underflow_start, ..._end]: contains .text sections
> + *		     for refcount error handling.
>   *
>   * Following global variables are optional and may be unavailable on some
>   * architectures and/or kernel configurations.
> @@ -39,6 +41,8 @@ extern char __per_cpu_load[], __per_cpu_start[], __per_cpu_end[];
>  extern char __kprobes_text_start[], __kprobes_text_end[];
>  extern char __entry_text_start[], __entry_text_end[];
>  extern char __start_rodata[], __end_rodata[];
> +extern char __refcount_overflow_start[], __refcount_overflow_end[];
> +extern char __refcount_underflow_start[], __refcount_underflow_end[];

I think this part is no longer needed, since you got rid of the new
sections?

> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 3558f4eb1a86..2f2f34942689 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -446,9 +446,18 @@
>  		ALIGN_FUNCTION();					\
>  		*(.text.hot .text .text.fixup .text.unlikely)		\
>  		*(.ref.text)						\
> +		REFCOUNT_TEXT						\
>  	MEM_KEEP(init.text)						\
>  	MEM_KEEP(exit.text)						\
>  
> +#define __REFCOUNT_TEXT(section)					\
> +		VMLINUX_SYMBOL(__##section##_start) = .;                \
> +		*(.text.##section)                                      \
> +		VMLINUX_SYMBOL(__##section##_end) = .;
> +
> +#define REFCOUNT_TEXT							\
> +	__REFCOUNT_TEXT(refcount_overflow)				\
> +	__REFCOUNT_TEXT(refcount_underflow)

Same here.

-- 
Josh