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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/07/15 04:16, Andy Lutomirski wrote:
> On Mon, Jul 27, 2015 at 7:20 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mon, Jul 27, 2015 at 9:18 AM, Boris Ostrovsky
>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> On 07/27/2015 11:53 AM, Andy Lutomirski wrote:
>>>> On Mon, Jul 27, 2015 at 8:36 AM, Boris Ostrovsky
>>>> <boris.ostrovsky@xxxxxxxxxx> wrote:
>>>>> On 07/25/2015 01:36 AM, Andy Lutomirski wrote:
>>>>>> Here's v3.  It fixes the "dazed and confused" issue, I hope.  It's also
>>>>>> probably a good general attack surface reduction, and it replaces some
>>>>>> scary code with IMO less scary code.
>>>>>>
>>>>>> Also, servers and embedded systems should probably turn off modify_ldt.
>>>>>> This makes that possible.
>>>>>>
>>>>>> Xen people, can you take a look at this?
>>>>>>
>>>>>> Willy and Kees: I left the config option alone.  The -tiny people will
>>>>>> like it, and we can always add a sysctl of some sort later.
>>>>>>
>>>>>> Changes from v3:
>>>>>>    - Hopefully fixed Xen.
>>>>>
>>>>> 32b-on-32b fails in the same manner. (but non-zero LDT is taken care of)
>>>>>
>>>>>>    - Fixed 32-bit test case on 32-bit native kernel.
>>>>>
>>>>> I am not sure I see what changed.
>>>> I misplaced the fix in the wrong git commit, so I failed to sent it.
>>>> Oops.
>>>>
>>>> I just sent v4.1 of patch 3.  Can you try that?
>>>
>>>
>>> I am hitting BUG() in Xen code (returning from a hypercall) when freeing LDT
>>> in destroy_context(). Interestingly though when I run the test in the
>>> debugger I get SIGILL (just like before) but no BUG().
>>>
>>> Let me get back to you on that later today.
>>>
>>>
>> 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.  If writeable aliases did exist then
xen_alloc_ldt() could indeed be insufficient to make the frames usable
as an LDT, but xen_free_ldt() wouldn't fail when trying to recreate the
writeable mappings.

~Andrew

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