Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934200AbbGVAHd (ORCPT ); Tue, 21 Jul 2015 20:07:33 -0400 Received: from mail-lb0-f170.google.com ([209.85.217.170]:36615 "EHLO mail-lb0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933883AbbGVAHc (ORCPT ); Tue, 21 Jul 2015 20:07:32 -0400 MIME-Version: 1.0 In-Reply-To: <55AED813.5020603@citrix.com> References: <55AEBF76.4040501@oracle.com> <55AED813.5020603@citrix.com> From: Andy Lutomirski Date: Tue, 21 Jul 2015 17:07:11 -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: 3328 Lines: 83 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? > > 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. > > 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. --Andy -- 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/