Date:	Tue, 28 Jul 2015 10:05:41 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Andy Lutomirski <luto@...capital.net>
CC:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config
 option

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)

-boris

> 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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