Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

[Andrew Cooper <andrew.cooper3@citrix.com>]

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:[<c10530f2>] 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]  [<c105312d>] xen_free_ldt+0x2d/0x40
>>>>>> [   53.986677]  [<c1062310>] free_ldt_struct.part.1+0x10/0x40
>>>>>> [   53.986677]  [<c1062735>] destroy_context+0x25/0x40
>>>>>> [   53.986677]  [<c10a764e>] __mmdrop+0x1e/0xc0
>>>>>> [   53.986677]  [<c10c9858>] finish_task_switch+0xd8/0x1a0
>>>>>> [   53.986677]  [<c1863736>] __schedule+0x316/0x950
>>>>>> [   53.986677]  [<c1863d96>] schedule+0x26/0x70
>>>>>> [   53.986677]  [<c10ac613>] do_wait+0x1b3/0x200
>>>>>> [   53.986677]  [<c10ac9d7>] SyS_waitpid+0x67/0xd0
>>>>>> [   53.986677]  [<c10aa820>] ? task_stopped_code+0x50/0x50
>>>>>> [   53.986677]  [<c186717a>] 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: [<c10530f2>] 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