From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option Date: Tue, 28 Jul 2015 16:23:33 +0100 Message-ID: <55B79E75.4010000__637.371213133697$1438097287$gmane$org@citrix.com> References: <55B64FEA.70204@oracle.com> <55B659EC.5030009@oracle.com>

<55B75993.90909@citrix.com> <55B78C35.1050702@oracle.com> <55B79314.8060009@citrix.com> <55B796BF.1080005@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <55B796BF.1080005@oracle.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Boris Ostrovsky , Andy Lutomirski Cc: "security@kernel.org" , Jan Beulich , Peter Zijlstra , X86 ML , "linux-kernel@vger.kernel.org" , Steven Rostedt , xen-devel , Borislav Petkov , Andy Lutomirski , Sasha Levin List-Id: xen-devel@lists.xenproject.org On 28/07/15 15:50, Boris Ostrovsky wrote: > 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. Are you certain that the set_ldt(NULL, 0) has been flushed to Xen to actually remove the LDT reference? All of this is hidden behind some lazy logic. > >> 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(). Did you get the companion "Bad L1 flags" error message with that? ~Andrew