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Date:   Wed, 19 Dec 2018 11:38:22 -0800
From:   Andi Kleen <ak@linux.intel.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
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Subject: Re: [RFC PATCH] x86/speculation: Don't inherit TIF_SSBD on execve()
Message-ID: <20181219193822.GU25620@tassilo.jf.intel.com>
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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