From: Dave Hansen <dave.hansen@linux.intel.com>

This is another case similar to what EFI does: create a new set of
page tables, map some code at a low address, and jump to it.  PTI
mistakes this low address for userspace and mistakenly marks it
non-executable in an effort to make it unusable for userspace.  Undo
the poison to allow execution.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ning Sun <ning.sun@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: tboot-devel@lists.sourceforge.net
Cc: linux-kernel@vger.kernel.org
---

 b/arch/x86/kernel/tboot.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c
--- a/arch/x86/kernel/tboot.c~pti-tboot-fix	2018-01-05 21:50:55.755554960 -0800
+++ b/arch/x86/kernel/tboot.c	2018-01-05 22:01:51.393553325 -0800
@@ -124,6 +124,13 @@ static int map_tboot_page(unsigned long
 	pte_t *pte;
 
 	pgd = pgd_offset(&tboot_mm, vaddr);
+	/*
+	 * PTI poisons low addresses in the kernel page tables in the
+	 * name of making them unusable for userspace.  To execute
+	 * code at such a low address, the poison must be cleared.
+	 */
+	pgd->pgd &= ~_PAGE_NX;
+
 	p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
 	if (!p4d)
 		return -1;
_