linux-kernel - Re: [RFC PATCH] x86/speculation: Don't inherit TIF

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Date:   Fri, 11 Jan 2019 20:52:31 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Waiman Long <longman@...hat.com>
cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Jiri Kosina <jikos@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH] x86/speculation: Don't inherit TIF_SSBD on
 execve()

On Wed, 19 Dec 2018, Waiman Long wrote:

> With the default SPEC_STORE_BYPASS_SECCOMP/SPEC_STORE_BYPASS_PRCTL mode,
> the TIF_SSBD bit will be inherited when a new task is fork'ed or cloned.
> 
> As only certain class of applications (like Java) requires disabling
> speculative store bypass for security purpose, it may not make sense to
> allow the TIF_SSBD bit to be inherited across execve() boundary where the
> new application may not need SSBD at all and is probably not aware that
> SSBD may have been turned on. This may cause an unnecessary performance
> loss of up to 20% in some cases.

Lot's of MAY's here. Aside of that this fundamentally changes the
behaviour. I'm not really a fan of doing that.

If there are good reasons to have a non-inherited variant, then we rather
introduce that instead of changing the existing semantics without a way for
existing userspace to notice.

Thanks,

	tglx