From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andy Lutomirski Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option Date: Wed, 29 Jul 2015 16:13:59 -0700 Message-ID: References: <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> <55B947AF.7020404@citrix.com> <55B94F9D.3000405@citrix.com> <55B957DE.60405@cantab.net> <55B95863.2000102@oracle.com> <55B95B70.8010902@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55B95B70.8010902@citrix.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andrew Cooper Cc: "security@kernel.org" , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , David Vrabel , Borislav Petkov , David Vrabel , Jan Beulich , Sasha Levin , Boris Ostrovsky List-Id: xen-devel@lists.xenproject.org On Wed, Jul 29, 2015 at 4:02 PM, Andrew Cooper wrote: > On 29/07/2015 23:49, Boris Ostrovsky wrote: >> On 07/29/2015 06:46 PM, David Vrabel wrote: >>> >>> On 29/07/2015 23:11, Andrew Cooper wrote: >>>> On 29/07/2015 23:05, Andy Lutomirski wrote: >>>>> On Wed, Jul 29, 2015 at 2:37 PM, Andrew Cooper >>>>> wrote: >>>>>> 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. >>>>> __get_user((char *)v, tmp), perhaps, unless there's something better >>>>> in the wings. Keep in mind that we need this for -stable, and it's >>>>> likely to get backported quite quickly due to CVE-2015-5157. >>>> Hmm - something like that tucked inside HYPERVISOR_update_va_mapping() >>>> would probably work, and certainly be minimal hassle for -stable. >>>> >>>> Altering the hypercall used is certainly not something to backport, nor >>>> are we sure it is a viable fix at this time. >>> Changing this one use of update_va_mapping to use mmu_update_normal_pt >>> is the correct fix to unblock this LDT series. I see no reason why this >>> cannot be backported. >> >> To properly fix it should include batching and that is not something >> that I think we should target for stable. > > Batching is absolutely not necessary to alter update_va_mapping to > mmu_update_normal_pt. After all, update_va_mapping isn't batched. > > However this isn't the first issue issue we have had lazy mmu faulting, > and I doubt it is the last. There are not many callsites of > update_va_mapping - I will audit them tomorrow and see if any similar > issues are lurking elsewhere. One thing I should add: nothing flushes old aliases in xen_alloc_ldt, yet I haven't been able to get xen_alloc_ldt to fail or subsequent LDT access to fault. Is this something we should be worried about? --Andy