Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932741AbbG1OwJ (ORCPT ); Tue, 28 Jul 2015 10:52:09 -0400 Received: from userp1040.oracle.com ([156.151.31.81]:29117 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932661AbbG1OwF (ORCPT ); Tue, 28 Jul 2015 10:52:05 -0400 Message-ID: <55B796BF.1080005@oracle.com> Date: Tue, 28 Jul 2015 10:50:39 -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> <55B78C35.1050702@oracle.com> <55B79314.8060009@citrix.com> In-Reply-To: <55B79314.8060009@citrix.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Source-IP: userv0021.oracle.com [156.151.31.71] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6262 Lines: 133 On 07/28/2015 10:35 AM, Andrew Cooper wrote: > On 28/07/15 15:05, Boris Ostrovsky wrote: >> 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) >> > In which case the page in question is still referenced in an LDT > (perhaps on a different vcpu) The problem is reproducible on a UP guest so it's not that. > or has been reused as a pagetable (I > really hope this is not the case). > > A sufficiently-debug Xen might be persuaded into telling you exactly > what it didn't like about the attempted transition. It just can't find l1 entry for the LDT address in __do_update_va_mapping(). -boris -- 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/