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

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Message-ID: <6bf08fce-cca2-5216-3fe8-8b5e7ab34b30@redhat.com>
Date:   Tue, 15 Jan 2019 10:54:56 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 01/15/2019 04:48 AM, Thomas Gleixner wrote:
> 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

Thanks for the advice. Will work on a v2 to be sent out later this week.

Cheers,
Longman