Re: [RFC PATCH] x86/speculation: Don't inherit TIF_SSBD on execve()

[Thomas Gleixner]

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