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: Thu, 30 Jul 2015 00:02:08 +0100 Message-ID: <55B95B70.8010902__36884.3892742463$1438211024$gmane$org@citrix.com> 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> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55B95863.2000102@oracle.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: Boris Ostrovsky , David Vrabel , Andy Lutomirski 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 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. ~Andrew