From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755235AbbG1DpF (ORCPT ); Mon, 27 Jul 2015 23:45:05 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:40188 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754745AbbG1DpD (ORCPT ); Mon, 27 Jul 2015 23:45:03 -0400 Message-ID: <55B6FA62.8010401@oracle.com> Date: Mon, 27 Jul 2015 23:43:30 -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: 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 , Andrew Cooper , 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> In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: aserv0021.oracle.com [141.146.126.233] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/27/2015 11:16 PM, Andy Lutomirski wrote: > On Mon, Jul 27, 2015 at 7:20 PM, Andy Lutomirski wrote: >> On Mon, Jul 27, 2015 at 9:18 AM, Boris Ostrovsky >> wrote: >>> On 07/27/2015 11:53 AM, Andy Lutomirski wrote: >>>> On Mon, Jul 27, 2015 at 8:36 AM, Boris Ostrovsky >>>> wrote: >>>>> On 07/25/2015 01:36 AM, Andy Lutomirski wrote: >>>>>> Here's v3. It fixes the "dazed and confused" issue, I hope. It's also >>>>>> probably a good general attack surface reduction, and it replaces some >>>>>> scary code with IMO less scary code. >>>>>> >>>>>> Also, servers and embedded systems should probably turn off modify_ldt. >>>>>> This makes that possible. >>>>>> >>>>>> Xen people, can you take a look at this? >>>>>> >>>>>> Willy and Kees: I left the config option alone. The -tiny people will >>>>>> like it, and we can always add a sysctl of some sort later. >>>>>> >>>>>> Changes from v3: >>>>>> - Hopefully fixed Xen. >>>>> >>>>> 32b-on-32b fails in the same manner. (but non-zero LDT is taken care of) >>>>> >>>>>> - Fixed 32-bit test case on 32-bit native kernel. >>>>> >>>>> I am not sure I see what changed. >>>> I misplaced the fix in the wrong git commit, so I failed to sent it. >>>> Oops. >>>> >>>> I just sent v4.1 of patch 3. Can you try that? >>> >>> >>> I am hitting BUG() in Xen code (returning from a hypercall) when freeing LDT >>> in destroy_context(). Interestingly though when I run the test in the >>> debugger I get SIGILL (just like before) but no BUG(). >>> >>> Let me get back to you on that later today. >>> >>> >> 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. Yes, this is as far as I got as well (I didn't try unaliasing but now that you found it -- it does indeed work). I am not sure whether you are saying this (I think you do, implicitly, since you are replacing vzalloc with kzalloc), but the problem only happens when we have multi-page LDT. And it is reproducible with a single CPU. -boris