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

[Waiman Long]

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?

Cheers,
Longman