x86/speculation: Add PR_SPEC_DISABLE_NOEXEC

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.
It will also remain when a new program is execve'ed.

Only certain class of applications (like Java) that can run on behalf of
multiple users on a single thread will require disabling speculative store
bypass for security purposes. Those applications will call prctl(2) at
startup time to disable SSB. They won't rely on the fact the SSB might have
been disabled. Other applications that don't need SSBD will just move on
without checking if SSBD has been turned on or not.

The fact that the TIF_SSBD is inherited across execve(2) boundary will
cause performance of applications that don't need SSBD but their
predecessors have SSBD on to be unwittingly impacted especially if they
write to memory a lot.

To remedy this problem, a new PR_SPEC_DISABLE_NOEXEC argument for the
PR_SET_SPECULATION_CTRL option of prctl(2) is added to allow applications
to specify that the SSBD feature bit on the task structure should be
cleared whenever a new program is being execve'ed.

Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Tim Chen <[email protected]>
Cc: KarimAllah Ahmed <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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