From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option Date: Wed, 29 Jul 2015 22:37:51 +0100 Message-ID: <55B947AF.7020404__16777.3508503617$1438205986$gmane$org@citrix.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andy Lutomirski , Boris Ostrovsky Cc: "security@kernel.org" , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , Borislav Petkov , David Vrabel , Jan Beulich , Sasha Levin List-Id: xen-devel@lists.xenproject.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