From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boris Ostrovsky Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option Date: Wed, 29 Jul 2015 18:40:28 -0400 Message-ID: <55B9565C.50003__28804.7845279381$1438209844$gmane$org@oracle.com> References: <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> <55B947AF.7020404@citrix.com> <55B94F9D.3000405@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55B94F9D.3000405@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 , 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 07/29/2015 06:11 PM, 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. OK, I'll test tonight this quick fix and will defer a more proper patch for 4.3 then. -boris