From mboxrd@z Thu Jan 1 00:00:00 1970 Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752495AbeAJVNZ (ORCPT + 1 other); Wed, 10 Jan 2018 16:13:25 -0500 Received: from mga06.intel.com ([134.134.136.31]:43843 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751328AbeAJVNY (ORCPT ); Wed, 10 Jan 2018 16:13:24 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,342,1511856000"; d="scan'208";a="18245612" Subject: [PATCH] x86/pti: unpoison pgd for trusted boot To: linux-kernel@vger.kernel.org Cc: x86@kernel.org, Dave Hansen , ning.sun@intel.com, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, tboot-devel@lists.sourceforge.net, aarcange@redhat.com, jcm@redhat.com, dwmw@amazon.co.uk, pbonzini@redhat.com, gnomes@lxorguk.ukuu.org.uk, torvalds@linux-foundation.org, andi@firstfloor.org, gregkh@linux-foundation.org, tim.c.chen@linux.intel.com, law@redhat.com, nickc@redhat.com, luto@kernel.org, peterz@infradead.org From: Dave Hansen Date: Wed, 10 Jan 2018 13:11:18 -0800 Message-Id: <20180110211118.A314040D@viggo.jf.intel.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: I believe this should replace 262b6b30087 in -tip. The patch in -tip potentially misses the pgd clearing if pud_alloc() sets a PGD. It would also be nice to have that comment back. Note that the -tip commit 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. -- From: Dave Hansen This is another case similar to what EFI does: create a new set of page tables, map some code at a low address, and jump to it. PTI mistakes this low address for userspace and mistakenly marks it non-executable in an effort to make it unusable for userspace. Undo the poison to allow execution. Signed-off-by: Dave Hansen Cc: Ning Sun Cc: Thomas Gleixner Cc: Ingo Molnar Cc: "H. Peter Anvin" Cc: x86@kernel.org Cc: tboot-devel@lists.sourceforge.net Cc: linux-kernel@vger.kernel.org Cc: Andrea Arcangeli CC: Jon Masters Cc: "Woodhouse, David" Cc: Paolo Bonzini Cc: Alan Cox Cc: Linus Torvalds Cc: Andi Kleen Cc: Greg Kroah-Hartman CC: "Tim Chen" Cc: Jeff Law Cc: Nick Clifton Cc: Andy Lutomirski Cc: Peter Zijlstra --- b/arch/x86/kernel/tboot.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff -puN arch/x86/kernel/tboot.c~pti-tboot-fix arch/x86/kernel/tboot.c --- a/arch/x86/kernel/tboot.c~pti-tboot-fix 2018-01-09 17:12:49.776734656 -0800 +++ b/arch/x86/kernel/tboot.c 2018-01-09 17:12:49.784734656 -0800 @@ -138,6 +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; } _