x86/ldt: Make modify_ldt synchronous

x86/ldt: Make modify_ldt synchronous

modify_ldt() has questionable locking and does not synchronize
threads.  Improve it: redesign the locking and synchronize all
threads' LDTs using an IPI on all modifications.

This will dramatically slow down modify_ldt in multithreaded
programs, but there shouldn't be any multithreaded programs that
care about modify_ldt's performance in the first place.

This fixes some fallout from the CVE-2015-5157 fixes.

Signed-off-by: Andy Lutomirski <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Cc: Andrew Cooper <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Jan Beulich <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Sasha Levin <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected] <[email protected]>
Cc: <[email protected]>
Cc: xen-devel <[email protected]>
Link: http://lkml.kernel.org/r/4c6978476782160600471bd865b318db34c7b628.1438291540.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>