From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752994AbbG1OH1 (ORCPT ); Tue, 28 Jul 2015 10:07:27 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:23723 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751323AbbG1OH0 (ORCPT ); Tue, 28 Jul 2015 10:07:26 -0400 Message-ID: <55B78C35.1050702@oracle.com> Date: Tue, 28 Jul 2015 10:05:41 -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: Andy Lutomirski , Peter Zijlstra , Steven Rostedt , "security@kernel.org" , X86 ML , Borislav Petkov , Sasha Levin , "linux-kernel@vger.kernel.org" , Konrad Rzeszutek Wilk , Jan Beulich , xen-devel Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option References: <55B64FEA.70204@oracle.com> <55B659EC.5030009@oracle.com> <55B75993.90909@citrix.com> In-Reply-To: <55B75993.90909@citrix.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: aserv0022.oracle.com [141.146.126.234] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/28/2015 06:29 AM, Andrew Cooper wrote: > >>> After forward-porting my virtio patches, I got this thing to run on >>> Xen. After several tries, I got: >>> >>> [ 53.985707] ------------[ cut here ]------------ >>> [ 53.986314] kernel BUG at arch/x86/xen/enlighten.c:496! >>> [ 53.986677] invalid opcode: 0000 [#1] SMP >>> [ 53.986677] Modules linked in: >>> [ 53.986677] CPU: 0 PID: 1400 Comm: bash Not tainted 4.2.0-rc4+ #4 >>> [ 53.986677] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), >>> BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org >>> 04/01/2014 >>> [ 53.986677] task: c2376180 ti: c0874000 task.ti: c0874000 >>> [ 53.986677] EIP: 0061:[] EFLAGS: 00010282 CPU: 0 >>> [ 53.986677] EIP is at set_aliased_prot+0xb2/0xc0 >>> [ 53.986677] EAX: ffffffea EBX: cc3d1000 ECX: 0672e063 EDX: 80000000 >>> [ 53.986677] ESI: 00000000 EDI: 80000000 EBP: c0875e94 ESP: c0875e74 >>> [ 53.986677] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0069 >>> [ 53.986677] CR0: 80050033 CR2: b77404d4 CR3: 020b6000 CR4: 00042660 >>> [ 53.986677] Stack: >>> [ 53.986677] 80000000 0672e063 000021c0 cc3d1000 00000001 cc3d2000 >>> 00000b4a 00000200 >>> [ 53.986677] c0875ea8 c105312d c2317940 c2373a80 00000000 c0875eb4 >>> c1062310 c01861c0 >>> [ 53.986677] c0875ec0 c1062735 c01861c0 c0875ed4 c10a764e c7007a00 >>> c2373a80 00000000 >>> [ 53.986677] Call Trace: >>> [ 53.986677] [] xen_free_ldt+0x2d/0x40 >>> [ 53.986677] [] free_ldt_struct.part.1+0x10/0x40 >>> [ 53.986677] [] destroy_context+0x25/0x40 >>> [ 53.986677] [] __mmdrop+0x1e/0xc0 >>> [ 53.986677] [] finish_task_switch+0xd8/0x1a0 >>> [ 53.986677] [] __schedule+0x316/0x950 >>> [ 53.986677] [] schedule+0x26/0x70 >>> [ 53.986677] [] do_wait+0x1b3/0x200 >>> [ 53.986677] [] SyS_waitpid+0x67/0xd0 >>> [ 53.986677] [] ? task_stopped_code+0x50/0x50 >>> [ 53.986677] [] syscall_call+0x7/0x7 >>> [ 53.986677] Code: e8 c1 e3 0c 81 eb 00 00 00 40 39 5d ec 74 11 8b >>> 4d e4 8b 55 e0 31 f6 e8 dd e0 fa ff 85 c0 75 0d 83 c4 14 5b 5e 5f 5d >>> c3 90 0f 0b <0f> 0b 0f 0b 8d 76 00 8d bc 27 00 00 00 00 85 d2 74 31 55 >>> 89 e5 >>> [ 53.986677] EIP: [] set_aliased_prot+0xb2/0xc0 SS:ESP 0069:c0875e74 >>> [ 54.010069] ---[ end trace 89ac35b29c1c59bb ]--- >>> >>> Is that the error you're seeing? >>> >>> If I change xen_free_ldt to: >>> >>> static void xen_free_ldt(struct desc_struct *ldt, unsigned entries) >>> { >>> const unsigned entries_per_page = PAGE_SIZE / LDT_ENTRY_SIZE; >>> int i; >>> >>> vm_unmap_aliases(); >>> xen_mc_flush(); >>> >>> for(i = 0; i < entries; i += entries_per_page) >>> set_aliased_prot(ldt + i, PAGE_KERNEL); >>> } >>> >>> then it works. I don't know why this makes a difference. >>> (xen_mc_flush makes a little bit of sense to me. vm_unmap_aliases >>> doesn't.) >>> >> That fix makes sense if there's some way that the vmalloc area we're >> freeing has an extra alias somewhere, which is very much possible. On >> the other hand, I don't see how this happens without first doing an >> MMUEXT_SET_LDT with an unexpectedly aliased address, and I would have >> expected that to blow up and/or result in test case failures. >> >> But I'm still confused, because it seems like Xen will never populate >> the actual (hidden) LDT mapping unless the pages backing it are >> unaliased and well-formed, which make me wonder why this stuff ever >> worked. Wouldn't LDT access with pre-existing vmalloc aliases result >> in segfaults? >> >> The semantics seem to be very odd. xen_free_ldt with an aliased >> address might fail (and OOPS), but actual access to the LDT with an >> aliased address page faults. >> >> Also, using kzalloc for everything fixes the problem, which suggests >> that there really is something to my theory that the problem involves >> unexpected aliases. > Xen does lazily populate the LDT frames. The first time a page is ever > referenced via the LDT, Xen will perform a typechange. > > Under Xen, guest mappings are reference counted with both a plain > reference, and a type count. Types of writeable, segdec and pagetables > are mutually exclusive. This prevents the guest from having writeable > mappings of interesting datastructures, but readable mappings are fine. > Typechanges may only occur when the type reference count is 0. > > At the point of the typechange, no writeable mappings of the frame may > exist (and it must not be referenced by a L2 or greater page directory), > or the typechange will fail. Additionally the descriptors are audited > at this point, so if Xen objects to any of the descriptors in the same > page, the typechange will also fail. > > If the typechange fails, the pagefault gets propagated back to the guest. > > The corollary to this is that, for xen_free_ldt() to create writeable > mappings again, a typechange back to writeable is needed. This will > fail if the LDT frames are still referenced in any vcpus LDT. > > It would be interesting to know which of the two BUG()s in > set_aliased_prot() tripped. The first one (i.e. not the alias) -boris > If writeable aliases did exist then > xen_alloc_ldt() could indeed be insufficient to make the frames usable > as an LDT, but xen_free_ldt() wouldn't fail when trying to recreate the > writeable mappings. > > ~Andrew