Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755545AbbGVCFo (ORCPT ); Tue, 21 Jul 2015 22:05:44 -0400 Received: from aserp1040.oracle.com ([141.146.126.69]:21877 "EHLO aserp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751756AbbGVCFm (ORCPT ); Tue, 21 Jul 2015 22:05:42 -0400 Message-ID: <55AEFA3C.3020006@oracle.com> Date: Tue, 21 Jul 2015 22:04:44 -0400 From: Boris Ostrovsky User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Andrew Cooper , Andy Lutomirski CC: "security@kernel.org" , Jan Beulich , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , Borislav Petkov , stable , Andy Lutomirski , Sasha Levin Subject: Re: [Xen-devel] [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous References: <55AEBF76.4040501@oracle.com> <55AED813.5020603@citrix.com> <55AEE21E.80108@citrix.com> <55AEE8A7.4040904@citrix.com> In-Reply-To: <55AEE8A7.4040904@citrix.com> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: userv0021.oracle.com [156.151.31.71] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4916 Lines: 120 On 07/21/2015 08:49 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. Why not wrap mm_context_t's ldt and size into a struct (just like ldt_struct but without __pad) and have a single allocation of ldt? I.e. struct ldt_struct { int size; struct desc_struct *entries; } --- a/arch/x86/include/asm/mmu.h +++ b/arch/x86/include/asm/mmu.h @@ -9,8 +9,7 @@ * we put the segment information here. */ typedef struct { - void *ldt; - int size; + struct ldt_struct ldt; #ifdef CONFIG_X86_64 /* True if mm supports a task running in 32 bit compatibility mode. */ -boris >>> 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. > > ~Andrew > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xen.org > http://lists.xen.org/xen-devel -- 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/