From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ppsw-50.csi.cam.ac.uk ([131.111.8.150]:50084 "EHLO ppsw-50.csi.cam.ac.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751690AbbGVAV4 (ORCPT ); Tue, 21 Jul 2015 20:21:56 -0400 Subject: Re: [PATCH v2 1/3] x86/ldt: Make modify_ldt synchronous To: Andy Lutomirski References: <55AEBF76.4040501@oracle.com> <55AED813.5020603@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 Message-ID: <55AEE21E.80108@citrix.com> Date: Wed, 22 Jul 2015 01:21:50 +0100 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org List-ID: 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. > >> I think I have a solution, but I doubt it is going to be very popular. >> >> * Make a new paravirt hook for allocation of ldt_struct, so the paravirt >> backend can choose an alignment if needed >> * Make absolutely certain that __pad has the value 0 (so size and __pad >> combined don't look like a present descriptor) >> * Never hand selector 0x0008 to unsuspecting users. > Yuck. I actually meant 0x0004, but yes. Very much yuck. > >> 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. ~Andrew