From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754170AbbG2ViK (ORCPT ); Wed, 29 Jul 2015 17:38:10 -0400 Received: from ppsw-50.csi.cam.ac.uk ([131.111.8.150]:35732 "EHLO ppsw-50.csi.cam.ac.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752273AbbG2ViI (ORCPT ); Wed, 29 Jul 2015 17:38:08 -0400 X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/ Subject: Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option To: Andy Lutomirski , Boris Ostrovsky References: <55B64FEA.70204@oracle.com> <55B659EC.5030009@oracle.com> <55B75993.90909@citrix.com> <55B7AE39.7000101@citrix.com> <55B7B791.2050208@oracle.com> <55B822B8.3090608@citrix.com> <55B841FF.2000102@oracle.com> <55B8E16C.2050406@citrix.com> <55B8E68B.2030305@oracle.com> <55B9236B.9090507@citrix.com> <55B94451.8040600@oracle.com> Cc: "security@kernel.org" , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , Borislav Petkov , Jan Beulich , Sasha Levin , David Vrabel , Konrad Wilk From: Andrew Cooper X-Enigmail-Draft-Status: N1110 Message-ID: <55B947AF.7020404@citrix.com> Date: Wed, 29 Jul 2015 22:37:51 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/07/2015 22:26, Andy Lutomirski wrote: > On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky > wrote: >> On 07/29/2015 03:03 PM, Andrew Cooper wrote: >>> On 29/07/15 15:43, Boris Ostrovsky wrote: >>>> FYI, I have got a repro now and am investigating. >>> Good and bad news. This bug has nothing to do with LDTs themselves. >>> >>> I have worked out what is going on, but this: >>> >>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c >>> index 5abeaac..7e1a82e 100644 >>> --- a/arch/x86/xen/enlighten.c >>> +++ b/arch/x86/xen/enlighten.c >>> @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v, pgprot_t prot) >>> pte = pfn_pte(pfn, prot); >>> + (void)*(volatile int*)v; >>> if (HYPERVISOR_update_va_mapping((unsigned long)v, pte, 0)) { >>> pr_err("set_aliased_prot va update failed w/ lazy mode >>> %u\n", paravirt_get_lazy_mode()); >>> BUG(); >>> >>> Is perhaps not the fix we are looking for, and every use of >>> HYPERVISOR_update_va_mapping() is susceptible to the same problem. >> >> I think in most cases we know that page is mapped so hopefully this is the >> only site that we need to be careful about. > Is there any chance we can get some kind of quick-and-dirty fix that > can go to x86/urgent in the next few days even if a clean fix isn't > available yet? Quick and dirty? Reading from v is the most obvious and quick way, for areas where we are certain v exists, is kernel memory and is expected to have a backing page. I don't know offhand how many of current HYPERVISOR_update_va_mapping() callsites this applies to. > >>> The update_va_mapping hypercall is designed to emulate writing the pte >>> for v, with auditing applied. As part of this, it does a pagewalk on v >>> to locate and map the l1. During this walk, Xen it finds the l2 not >>> present, and fails the hypercall. i.e. v is not reachable from the >>> current cr3. >>> >>> Reading the virtual address immediately before issuing the hypercall >>> causes Linux's memory faulting logic to fault in the l2. This also >>> explains why vm_unmap_aliases() appears to fix the issue; it is likely >>> to fault in enough of the paging structure for v to be reachable. >> >> We've just touched this page (in write_ldt()) in this test so why would it >> not be mapped? > With my patches applied, the LDT is never written via any paravirt > hook -- I write it once (possibly implicitly using kzalloc/vzalloc) > before paravirt_alloc_ldt(), and write_ldt() is never called. We > could even remove it write_ldt() :) Even better! ~Andrew