Commit 445b69e3 authored Jan 10, 2018 by Dave Hansen Committed by Thomas Gleixner Jan 11, 2018

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: 262b6b30 ("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

parent 612e8e93

arch/x86/kernel/tboot.c

+11 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

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