linux-kernel - [tip:x86/pti] x86/pti: Make unpoison of pgd for trusted boot work for real

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Message-ID: <tip-445b69e3b75e42362a5bdc13c8b8f61599e2228a@git.kernel.org>
Date:   Thu, 11 Jan 2018 15:22:03 -0800
From:   tip-bot for Dave Hansen <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     mingo@...nel.org, aarcange@...hat.com, hpa@...or.com,
        jcm@...hat.com, linux-kernel@...r.kernel.org,
        dave.hansen@...ux.intel.com, tim.c.chen@...ux.intel.com,
        tglx@...utronix.de
Subject: [tip:x86/pti] x86/pti: Make unpoison of pgd for trusted boot work
 for real

Commit-ID:  445b69e3b75e42362a5bdc13c8b8f61599e2228a
Gitweb:     https://git.kernel.org/tip/445b69e3b75e42362a5bdc13c8b8f61599e2228a
Author:     Dave Hansen <dave.hansen@...ux.intel.com>
AuthorDate: Wed, 10 Jan 2018 14:49:39 -0800
Committer:  Thomas Gleixner <tglx@...utronix.de>
CommitDate: Thu, 11 Jan 2018 23:36:59 +0100

x86/pti: Make unpoison of pgd for trusted boot work for real

The inital fix for trusted boot and PTI potentially misses the pgd clearing
if pud_alloc() sets a PGD.  It probably works in *practice* because for two
adjacent calls to map_tboot_page() that share a PGD entry, the first will
clear NX, *then* allocate and set the PGD (without NX clear).  The second
call will *not* allocate but will clear the NX bit.

Defer the NX clearing to a point after it is known that all top-level
allocations have occurred.  Add a comment to clarify why.

[ tglx: Massaged changelog ]

Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Reviewed-by: Andrea Arcangeli <aarcange@...hat.com>
Cc: Jon Masters <jcm@...hat.com>
Cc: "Tim Chen" <tim.c.chen@...ux.intel.com>
Cc: gnomes@...rguk.ukuu.org.uk
Cc: peterz@...radead.org
Cc: ning.sun@...el.com
Cc: tboot-devel@...ts.sourceforge.net
Cc: andi@...stfloor.org
Cc: luto@...nel.org
Cc: law@...hat.com
Cc: pbonzini@...hat.com
Cc: torvalds@...ux-foundation.org
Cc: gregkh@...ux-foundation.org
Cc: dwmw@...zon.co.uk
Cc: nickc@...hat.com
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com
---
 arch/x86/kernel/tboot.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index 75869a4..a2486f4 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -127,7 +127,6 @@ 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;
@@ -139,6 +138,17 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,
 		return -1;
 	set_pte_at(&tboot_mm, vaddr, pte, pfn_pte(pfn, prot));
 	pte_unmap(pte);
+
+	/*
+	 * 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.
+	 *
+	 * Note: 'pgd' actually gets set in p4d_alloc() _or_
+	 * pud_alloc() depending on 4/5-level paging.
+	 */
+	pgd->pgd &= ~_PAGE_NX;
+
 	return 0;
 }