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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/28/2015 11:15 AM, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 28, 2015 at 10:50:39AM -0400, 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.
> The Linux kernel does a bunch of lazy maps and unmaps and we
> may be getting an interrupt while the lazy unmap hasn't been
> called  (arch_leave_lazy_mmu_mode).
>
> Having the vm_unmap_aliases and then xc_mc_flush (which is what
> arch_leave_lazy_mmu_mode ends up doing too and more) would solve it.
>
> Thought I would have thought that vm_unmap_aliases would call
> arch_leave_lazy_mmu_mode.
>>> 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().
> Which would imply that it has not been written in. Which corresponds
> to the set_aliased_prot hitting the first BUG_ON.
>
> The xc_mc_flush() also triggers the batched hypercalls - which means we
> may have some hypercalls that have not yet gone to the hypervisor and
> then we try do an LDT hypercall (not batched).

If this were true then having xen_mc_flush() in xen_free_ldt() would 
have fixed this problem, and it didn't (without preceding 
vm_unmap_aliases()).

In any case, the patch below doesn't help.

-boris

>
> You could try building with this debug:
>
>
> diff --git a/arch/x86/xen/multicalls.c b/arch/x86/xen/multicalls.c
> index ea54a08..5d214ce 100644
> --- a/arch/x86/xen/multicalls.c
> +++ b/arch/x86/xen/multicalls.c
> @@ -28,9 +28,9 @@
>   #include "multicalls.h"
>   #include "debugfs.h"
>   
> -#define MC_BATCH	32
> +#define MC_BATCH	1
>   
> -#define MC_DEBUG	0
> +#define MC_DEBUG	1
>   
>   #define MC_ARGS		(MC_BATCH * 16)
>   
>> -boris
>>