From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753928AbbG2VZK (ORCPT ); Wed, 29 Jul 2015 17:25:10 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:48300 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753309AbbG2VZI (ORCPT ); Wed, 29 Jul 2015 17:25:08 -0400 Message-ID: <55B94451.8040600@oracle.com> Date: Wed, 29 Jul 2015 17:23:29 -0400 From: Boris Ostrovsky User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Andrew Cooper , Andy Lutomirski 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 Subject: Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option 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> In-Reply-To: <55B9236B.9090507@citrix.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. > > 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? > > One solution might be to use MMU_NORMAL_PT_UPDATE hypercall instead, > which take the physical address of pte to update. This won't fail in > Xen if part of the paging structure is missing, and can be batched. Yes, it does work. Thanks Andrew. -boris