linux-kernel - Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

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Date:   Sat, 6 Jan 2018 10:35:41 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     Andi Kleen <ak@...ux.intel.com>, Paul Turner <pjt@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>, tglx@...utronix.de,
        Kees Cook <keescook@...gle.com>,
        Rik van Riel <riel@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Jiri Kosina <jikos@...nel.org>, gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v5 02/12] x86/retpoline: Add initial retpoline support

On Sat, Jan 06, 2018 at 11:49:24AM +0000, David Woodhouse wrote:
> +/*
> + * NOSPEC_JMP and NOSPEC_CALL macros can be used instead of a simple
> + * indirect jmp/call which may be susceptible to the Spectre variant 2
> + * attack.
> + */

Can be, or must be?

> +.macro NOSPEC_JMP reg:req
> +#ifdef CONFIG_RETPOLINE
> +	ALTERNATIVE_2 __stringify(jmp *\reg),				\
> +		__stringify(RETPOLINE_JMP \reg), X86_FEATURE_RETPOLINE,	\
> +		__stringify(lfence; jmp *\reg), X86_FEATURE_RETPOLINE_AMD
> +#else
> +	jmp	*\reg
> +#endif
> +.endm
> +
> +.macro NOSPEC_CALL reg:req
> +#ifdef CONFIG_RETPOLINE
> +	ALTERNATIVE_2 __stringify(call *\reg),				\
> +		__stringify(RETPOLINE_CALL \reg), X86_FEATURE_RETPOLINE,\
> +		__stringify(lfence; call *\reg), X86_FEATURE_RETPOLINE_AMD
> +#else
> +	call	*\reg
> +#endif
> +.endm

Would it make any sense to name these INDIRECT_JMP and INDIRECT_CALL instead?
NOSPEC_ seems to describe how it needs to be implemented on some CPUs, as
opposed to what the user wants to do (make an indirect jump or call).

Eric