[PATCH 4.9] x86/pti: Make unpoison of pgd for trusted boot work for real

[PATCH 4.9] x86/pti: Make unpoison of pgd for trusted boot work for real

[Hugh Dickins]

From: Dave Hansen <dave.hansen@linux.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 ]

Fixes: 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: peterz@infradead.org
Cc: ning.sun@intel.com
Cc: tboot-devel@lists.sourceforge.net
Cc: andi@firstfloor.org
Cc: luto@kernel.org
Cc: law@redhat.com
Cc: pbonzini@redhat.com
Cc: torvalds@linux-foundation.org
Cc: gregkh@linux-foundation.org
Cc: dwmw@amazon.co.uk
Cc: nickc@redhat.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Hugh Dickins <hughd@google.com>

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

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index 8402907825b0..21454e254a4c 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -134,6 +134,16 @@ 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 pud_alloc().
+	 */
+	pgd->pgd &= ~_PAGE_NX;
+
 	return 0;
 }
 
-- 
2.16.0.rc1.238.g530d649a79-goog

Patch "x86/pti: Make unpoison of pgd for trusted boot work for real" has been added to the 4.9-stable tree

[gregkh]

This is a note to let you know that I've just added the patch titled

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

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-pti-make-unpoison-of-pgd-for-trusted-boot-work-for-real.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From hughd@google.com  Mon Feb  5 04:59:38 2018
From: Hugh Dickins <hughd@google.com>
Date: Mon, 29 Jan 2018 18:17:26 -0800
Subject: x86/pti: Make unpoison of pgd for trusted boot work for real
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>, Jon Masters <jcm@redhat.com>, Tim Chen <tim.c.chen@linux.intel.com>, gnomes@lxorguk.ukuu.org.uk, peterz@infradead.org, ning.sun@intel.com, tboot-devel@lists.sourceforge.net, andi@firstfloor.org, luto@kernel.org, law@redhat.com, pbonzini@redhat.com, torvalds@linux-foundation.org, gregkh@linux-foundation.org, dwmw@amazon.co.uk, nickc@redhat.com, stable@vger.kernel.org, Jiri Kosina <jkosina@suse.cz>, Hugh Dickins <hughd@google.com>
Message-ID: <20180130021726.229367-1-hughd@google.com>

From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jon Masters <jcm@redhat.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: gnomes@lxorguk.ukuu.org.uk
Cc: peterz@infradead.org
Cc: ning.sun@intel.com
Cc: tboot-devel@lists.sourceforge.net
Cc: andi@firstfloor.org
Cc: luto@kernel.org
Cc: law@redhat.com
Cc: pbonzini@redhat.com
Cc: torvalds@linux-foundation.org
Cc: gregkh@linux-foundation.org
Cc: dwmw@amazon.co.uk
Cc: nickc@redhat.com
Link: https://lkml.kernel.org/r/20180110224939.2695CD47@viggo.jf.intel.com
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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;
 }
 


Patches currently in stable-queue which might be from hughd@google.com are

queue-4.9/kaiser-allocate-pgd-with-order-0-when-pti-off.patch
queue-4.9/x86-pti-make-unpoison-of-pgd-for-trusted-boot-work-for-real.patch
queue-4.9/kaiser-fix-intel_bts-perf-crashes.patch