Patch "x86/pti: Make unpoison of pgd for trusted boot work for real" has been added to the 4.14-stable tree

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 4.14-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-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Dave Hansen <[email protected]>
Date: Wed, 10 Jan 2018 14:49:39 -0800
Subject: x86/pti: Make unpoison of pgd for trusted boot work for real

From: Dave Hansen <[email protected]>

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 ]

---
 arch/x86/kernel/tboot.c |   12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

@@ -127,7 +127,6 @@ static int map_tboot_page(unsigned long
 	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
 		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;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-pti-unbreak-efi-old_memmap.patch queue-4.14/x86-documentation-add-pti-description.patch queue-4.14/x86-tboot-unbreak-tboot-with-pti-enabled.patch queue-4.14/x86-pti-make-unpoison-of-pgd-for-trusted-boot-work-for-real.patch