Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964891AbbGVBGn (ORCPT ); Tue, 21 Jul 2015 21:06:43 -0400 Received: from mail-lb0-f179.google.com ([209.85.217.179]:32960 "EHLO mail-lb0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934154AbbGVBGk (ORCPT ); Tue, 21 Jul 2015 21:06:40 -0400 MIME-Version: 1.0 In-Reply-To: <55AEE8A7.4040904@citrix.com> References: <55AEBF76.4040501@oracle.com> <55AED813.5020603@citrix.com> <55AEE21E.80108@citrix.com> <55AEE8A7.4040904@citrix.com> From: Andy Lutomirski Date: Tue, 21 Jul 2015 18:06:19 -0700 Message-ID: Subject: Re: [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous To: Andrew Cooper Cc: Boris Ostrovsky , Andy Lutomirski , Peter Zijlstra , Steven Rostedt , "security@kernel.org" , X86 ML , Borislav Petkov , Sasha Levin , "linux-kernel@vger.kernel.org" , Konrad Rzeszutek Wilk , stable , Jan Beulich , xen-devel Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4418 Lines: 100 On Tue, Jul 21, 2015 at 5:49 PM, Andrew Cooper wrote: > On 22/07/2015 01:28, Andy Lutomirski wrote: >> On Tue, Jul 21, 2015 at 5:21 PM, Andrew Cooper >> wrote: >>> On 22/07/2015 01:07, Andy Lutomirski wrote: >>>> On Tue, Jul 21, 2015 at 4:38 PM, Andrew Cooper >>>> wrote: >>>>> On 21/07/2015 22:53, Boris Ostrovsky wrote: >>>>>> On 07/21/2015 03:59 PM, Andy Lutomirski wrote: >>>>>>> --- a/arch/x86/include/asm/mmu_context.h >>>>>>> +++ b/arch/x86/include/asm/mmu_context.h >>>>>>> @@ -34,6 +34,44 @@ static inline void load_mm_cr4(struct mm_struct >>>>>>> *mm) {} >>>>>>> #endif >>>>>>> /* >>>>>>> + * ldt_structs can be allocated, used, and freed, but they are never >>>>>>> + * modified while live. >>>>>>> + */ >>>>>>> +struct ldt_struct { >>>>>>> + int size; >>>>>>> + int __pad; /* keep the descriptors naturally aligned. */ >>>>>>> + struct desc_struct entries[]; >>>>>>> +}; >>>>>> >>>>>> This breaks Xen which expects LDT to be page-aligned. Not sure why. >>>>>> >>>>>> Jan, Andrew? >>>>> PV guests are not permitted to have writeable mappings to the frames >>>>> making up the GDT and LDT, so it cannot make unaudited changes to >>>>> loadable descriptors. In particular, for a 32bit PV guest, it is only >>>>> the segment limit which protects Xen from the ring1 guest kernel. >>>>> >>>>> A lot of this code hasn't been touched in years, and it certainly >>>>> predates me. The alignment requirement appears to come from the virtual >>>>> region Xen uses to map the guests GDT and LDT. Strict alignment is >>>>> required for the GDT so Xen's descriptors starting at 0xe0xx are >>>>> correct, but the LDT alignment seems to be a side effect of similar >>>>> codepaths. >>>>> >>>>> For an LDT smaller than 8192 entries, I can't see any specific reason >>>>> for enforcing alignment, other than "that's the way it has always been". >>>>> >>>>> However, the guest would still have to relinquish write access to all >>>>> frames which make up the LDT, which looks to be a bit of an issue given >>>>> the snippet above. >>>> Does the LDT itself need to be aligned or just the address passed to >>>> paravirt_alloc_ldt? >>> The address which Xen receives needs to be aligned. >>> >>> It looks like xen_alloc_ldt() blindly assumes that the desc_struct *ldt >>> it is passed is page aligned, and passes it straight through. >> xen_alloc_ldt is just fiddling with protection though, I think. Isn't >> it xen_set_ldt that's the meat? We could easily pass xen_alloc_ldt a >> pointer to the ldt_struct. > > So it is. It is the linear_addr in xen_set_ldt() which Xen currently > audits to be page aligned. > >>>>> This will allow ldt_struct itself to be page aligned, and for the size >>>>> field to sit across the base/limit field of what would logically be >>>>> selector 0x0008 There would be some issues accessing size. To load >>>>> frames as an LDT, a guest must drop all refs to the page so that its >>>>> type may be changed from writeable to segdesc. After that, an >>>>> update_descriptor hypercall can be used to change size, and I believe >>>>> the guest may subsequently recreate read-only mappings to the frames in >>>>> question (although frankly it is getting late so you will want to double >>>>> check all of this). >>>>> >>>>> Anyhow, this looks like an issue which should be fixed up with slightly >>>>> more PVOps, rather than enforcing a Xen view of the world on native Linux. >>>>> >>>> I could presumably make the allocation the other way around so the >>>> size is at the end. I could even use two separate allocations if >>>> needed. >>> I suspect two separate allocations would be the better solution, as it >>> means that the size field doesn't need to be subject to funny page >>> permissions. >> True. OTOH we never write to the size field after allocating the thing. > > Right, but even reading it is going to cause problems if one of the > paravirt ops can't re-establish ro mappings. Does paravirt_alloc_ldt completely deny access or does it just set it RO? --Andy > > ~Andrew -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/