linux-kernel - Re: [PATCH v8.1 12/12] x86/retpoline: Fill return stack buffer on vmexit

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Date:   Fri, 12 Jan 2018 12:15:12 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
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>,
        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,
        x86@...nel.org, thomas.lendacky@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v8.1 12/12] x86/retpoline: Fill return stack buffer on
 vmexit

On Fri, 12 Jan 2018, David Woodhouse wrote:

> In accordance with the Intel and AMD documentation, we need to overwrite
> all entries in the RSB on exiting a guest, to prevent malicious branch
> target predictions from affecting the host kernel. This is needed both
> for retpoline and for IBRS.
> 
> [ak: numbers again for the RSB stuffing labels]
> Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> Tested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
> I love the smell of bikeshed paint in the morning. But to be fair, this
> one was actually an issue which might possibly have bitten in the future.
> 
> Can we please stop arguing about asm labels now though? Let's get this
> stuff done, and we can set about the oh-so-important task of persuading
> Linus to eliminate all numeric labels and rely on human-readable labels
> with %= and \@ to make them unique, some time after the dust settles.

Fair enough. I surely like the below way more than the sloppy hackery from
Andi which completely removed any form of documentation.

> +#define __FILL_RETURN_BUFFER(reg, nr, sp)	\
> +	mov	$(nr/2), reg;			\
> +771:						\
> +	call	772f;				\
> +773:	/* speculation trap */			\
> +	pause;					\
> +	jmp	773b;				\
> +772:						\
> +	call	774f;				\
> +775:	/* speculation trap */			\
> +	pause;					\
> +	jmp	775b;				\
> +774:						\
> +	dec	reg;				\
> +	jnz	771b;				\
> +	add	$(BITS_PER_LONG/8) * nr, sp;
> +