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

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Message-ID: <20181219193822.GU25620@tassilo.jf.intel.com>
Date:   Wed, 19 Dec 2018 11:38:22 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>, x86@...nel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.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>
Subject: Re: [RFC PATCH] x86/speculation: Don't inherit TIF_SSBD on execve()

On Wed, Dec 19, 2018 at 02:09:50PM -0500, 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.
> 
> The arch_setup_new_exec() function is updated to clear the TIF_SSBD
> bit unless it has been force-disabled.

This makes it impossible to write a wrapper that turns this mode
on for unmodified programs.

Do you have a real use case where this behavior is a problem?

-Andi