linux-kernel - Re: [PATCH] x86/bugs: protect against userspace-userspace spectreRSB

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Date:   Wed, 25 Jul 2018 08:45:12 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Borislav Petkov <bp@...e.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/bugs: protect against userspace-userspace spectreRSB

On Tue, Jul 24, 2018 at 09:53:30PM +0200, Jiri Kosina wrote:
> From: Jiri Kosina <jkosina@...e.cz>
> 
> The article "Spectre Returns! Speculation Attacks using the Return Stack
> Buffer" [1] describes two new (sub-)variants of spectrev2-like attack,
> making use solely of the RSB contents even on CPUs that don't fallback to
> BTB on RSB underflow (Skylake+).
> 
> Mitigate userspace-userspace attacks by always unconditionally filling RSB on
> context switch when generic spectrev2 mitigation has been enabled.
> 
> [1] https://arxiv.org/pdf/1807.07940.pdf
> 
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>

While I generally agree with this patch, isn't it odd that we would do
RSB filling on every context switch, but almost never do IBPB?

> +	 *	- RSB underflow (and switch to BTB) on Skylake+
> +	 *	- sepctreRSB variant of spectre v2 on X86_BUG_SPECTRE_V2 CPUs

"SpectreRSB"

>  	 */
> -	if ((!boot_cpu_has(X86_FEATURE_PTI) &&
> -	     !boot_cpu_has(X86_FEATURE_SMEP)) || is_skylake_era()) {
> -		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> -		pr_info("Spectre v2 mitigation: Filling RSB on context switch\n");
> -	}
> +	setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
> +	pr_info("Spectre v2 / spectreRSB mitigation: Filling RSB on context switch\n");

"SpectreRSB" (capitalized)

-- 
Josh