linux-kernel - [PATCH 4.9 16/92] x86/pti: Make unpoison of pgd for trusted boot work for real

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Date:   Fri,  9 Feb 2018 14:38:45 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Jon Masters <jcm@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        gnomes@...rguk.ukuu.org.uk, peterz@...radead.org,
        ning.sun@...el.com, tboot-devel@...ts.sourceforge.net,
        andi@...stfloor.org, luto@...nel.org, law@...hat.com,
        pbonzini@...hat.com, torvalds@...ux-foundation.org,
        dwmw@...zon.co.uk, nickc@...hat.com, Jiri Kosina <jkosina@...e.cz>,
        Hugh Dickins <hughd@...gle.com>, gregkh@...ux-foundation.org
Subject: [PATCH 4.9 16/92] x86/pti: Make unpoison of pgd for trusted boot work for real

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

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

commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream

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 ]

[ hughd notes: I have not tested tboot, but this looks to me as necessary
and as safe in old-Kaiser backports as it is upstream; I'm not submitting
the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e,
and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.]

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
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com
Cc: Jiri Kosina <jkosina@...e.cz>
Signed-off-by: Hugh Dickins <hughd@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/x86/kernel/tboot.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -134,6 +134,16 @@ static int map_tboot_page(unsigned long
 		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 pud_alloc().
+	 */
+	pgd->pgd &= ~_PAGE_NX;
+
 	return 0;
 }