Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753115AbeAJBLm (ORCPT + 1 other); Tue, 9 Jan 2018 20:11:42 -0500 Received: from mga07.intel.com ([134.134.136.100]:44195 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751410AbeAJBLl (ORCPT ); Tue, 9 Jan 2018 20:11:41 -0500 X-Amp-Result: UNSCANNABLE X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,337,1511856000"; d="scan'208";a="8580077" Subject: Re: Avoid speculative indirect calls in kernel To: Thomas Gleixner , Andrea Arcangeli References: <20180104015920.1ad7b9d3@alans-desktop> <1515054014.12987.75.camel@amazon.co.uk> <403e65be-cfd1-fd08-0401-2e26470b63d4@redhat.com> <4dde456c-fd15-e768-8876-5844c8b7c455@redhat.com> <9976a670-a023-ea1f-3f13-ee5253092533@redhat.com> <20180108102805.GK25546@redhat.com> <20180108213223.GF4703@redhat.com> Cc: Jon Masters , "Woodhouse, David" , Paolo Bonzini , Alan Cox , Linus Torvalds , Andi Kleen , Greg Kroah-Hartman , Tim Chen , Linux Kernel Mailing List , Jeff Law , Nick Clifton , Andy Lutomirski , Peter Zijlstra From: Dave Hansen Message-ID: <24e30389-00a5-b4ee-9610-fa70ebf1cea6@intel.com> Date: Tue, 9 Jan 2018 17:11:39 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/mixed; boundary="------------35F98FFB66E3F934AB7BF111" Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Return-Path: This is a multi-part message in MIME format. --------------35F98FFB66E3F934AB7BF111 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit On 01/09/2018 04:45 PM, Thomas Gleixner wrote: > On Mon, 8 Jan 2018, Andrea Arcangeli wrote: >> On Mon, Jan 08, 2018 at 09:53:02PM +0100, Thomas Gleixner wrote: >> Did my best to do the cleanest patch for tip, but I now figured Dave's >> original comment was spot on: a _PAGE_NX clear then becomes necessary >> also after pud_alloc not only after p4d_alloc. >> >> pmd_alloc would run into the same with x86 32bit non-PAE too. non-PAE doesn't have an NX bit. :) But we #define _PAGE_NX down to 0 there so it's harmless. >> So there are two choices, either going back to one single _PAGE_NX >> clear from the original Dave's original patch as below, or to add >> multiple clear after each level which was my objective and is more >> robust, but it may be overkill in this case. As long as it was one >> line it looked a clear improvement. >> >> Considering the caller in both cases is going to abort I guess we can >> use the one liner approach as Dave and Jiri did originally. > > Dave ? I agree with Andrea. 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. The patch I think we want is attached. --------------35F98FFB66E3F934AB7BF111 Content-Type: text/x-patch; name="pti-tboot-fix.patch" Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename="pti-tboot-fix.patch" 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 --- 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 2018-01-05 21:50:55.755554960 -0800 +++ b/arch/x86/kernel/tboot.c 2018-01-05 23:51:41.368536890 -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; } _ --------------35F98FFB66E3F934AB7BF111--