x86/entry/64: Dont use IST entry for #BP stack

x86/entry/64: Dont use IST entry for #BP stack

There's nothing IST-worthy about #BP/int3.  We don't allow kprobes
in the small handful of places in the kernel that run at CPL0 with
an invalid stack, and 32-bit kernels have used normal interrupt
gates for #BP forever.

Furthermore, we don't allow kprobes in places that have usergs while
in kernel mode, so "paranoid" is also unnecessary.

OraBug: 27895315
CVE: CVE-2018-8897

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit d8ba61ba58c88d5207c1ba2f7d9a2280e7d03be9)

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>