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@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

1 files changed, 11 insertions, 1 deletions

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index 75869a4b6c41..a2486f444073 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;
 }