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

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Message-ID: <alpine.DEB.2.21.1901151045580.1865@nanos.tec.linutronix.de>
Date:   Tue, 15 Jan 2019 10:48:48 +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 Mon, 14 Jan 2019, Waiman Long wrote:
> On 01/11/2019 02:52 PM, Thomas Gleixner wrote:
> > 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.
> 
> I understand your point. How about adding a ",noexec" auxillary option
> to the spec_store_bypass_disable command line to activate this new
> behavior without changing the default. Will that be acceptable?

I'd rather have an explicit PR_SPEC_DISABLE_NOEXEC argument for the PRCTL
so you can decide at the application level what kind of behaviour you want.

Thanks,

	tglx