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

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

From: Dave Hansen <dave.han...@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.han...@linux.intel.com>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Reviewed-by: Andrea Arcangeli <aarca...@redhat.com>
Cc: Jon Masters <j...@redhat.com>
Cc: "Tim Chen" <tim.c.c...@linux.intel.com>
Cc: gno...@lxorguk.ukuu.org.uk
Cc: pet...@infradead.org
Cc: ning....@intel.com
Cc: tboot-de...@lists.sourceforge.net
Cc: a...@firstfloor.org
Cc: l...@kernel.org
Cc: l...@redhat.com
Cc: pbonz...@redhat.com
Cc: torva...@linux-foundation.org
Cc: gre...@linux-foundation.org
Cc: d...@amazon.co.uk
Cc: ni...@redhat.com
Cc: sta...@vger.kernel.org
Link: https://lkml.kernel.org/r/20180110224939.2695c...@viggo.jf.intel.com
Signed-off-by: Hugh Dickins <hu...@google.com>
Signed-off-by: Greg Kroah-Hartman <gre...@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
@@ -140,6 +140,16 @@ 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;
 }
 


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