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

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

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) 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.

~Andrew