[tip:x86/pti] x86/tboot: Unbreak tboot with PTI enabled

[tip-bot for Dave Hansen]

Commit-ID:  262b6b30087246abf09d6275eb0c0dc421bcbe38
Gitweb:     https://git.kernel.org/tip/262b6b30087246abf09d6275eb0c0dc421bcbe38
Author:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
AuthorDate: Sat, 6 Jan 2018 18:41:14 +0100
Committer:  Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitDate: Mon, 8 Jan 2018 17:29:18 +0100

x86/tboot: Unbreak tboot with PTI enabled

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.

Fixes: 385ce0ea4c07 ("x86/mm/pti: Add Kconfig")
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>
Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Cc: Jon Masters <jcm@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Andi Kleen <andi@xxxxxxxxxxxxxx>
Cc: Jeff Law <law@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxxx>
Cc: David" <dwmw@xxxxxxxxxxxx>
Cc: Nick Clifton <nickc@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20180108102805.GK25546@xxxxxxxxxx
---
 arch/x86/kernel/tboot.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index a4eb279..75869a4 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -127,6 +127,7 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
 	p4d = p4d_alloc(&tboot_mm, pgd, vaddr);
 	if (!p4d)
 		return -1;
+	pgd->pgd &= ~_PAGE_NX;
 	pud = pud_alloc(&tboot_mm, p4d, vaddr);
 	if (!pud)
 		return -1;