Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964896AbbGVAt6 (ORCPT ); Tue, 21 Jul 2015 20:49:58 -0400 Received: from ppsw-50.csi.cam.ac.uk ([131.111.8.150]:58154 "EHLO ppsw-50.csi.cam.ac.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934061AbbGVAtz (ORCPT ); Tue, 21 Jul 2015 20:49:55 -0400 X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: http://www.cam.ac.uk/cs/email/scanner/ Subject: Re: [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous To: Andy Lutomirski References: <55AEBF76.4040501@oracle.com> <55AED813.5020603@citrix.com> <55AEE21E.80108@citrix.com> 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 From: Andrew Cooper X-Enigmail-Draft-Status: N1110 Message-ID: <55AEE8A7.4040904@citrix.com> Date: Wed, 22 Jul 2015 01:49:43 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4113 Lines: 87 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. ~Andrew -- 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/