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Commit ae1fc8de authored by Dave Hansen's avatar Dave Hansen Committed by Greg Kroah-Hartman
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x86/pti: Make unpoison of pgd for trusted boot work for real 



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: default avatarDave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarAndrea 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: default avatarHugh Dickins <hughd@google.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 0a61cd6c

arch/x86/kernel/tboot.c

+10 −0
Original line number Diff line number Diff line
@@ -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;

}