linux-kernel - Re: Is espfix64's double-fault thing OK on Xen?

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Date:	Mon, 14 Jul 2014 20:20:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Is espfix64's double-fault thing OK on Xen?

On Mon, Jul 14, 2014 at 7:46 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Mon, Jul 14, 2014 at 3:23 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 07/14/2014 02:35 PM, Andy Lutomirski wrote:
>>> Presumably the problem is here:
>>>
>>> ENTRY(xen_iret)
>>>     pushq $0
>>> 1:    jmp hypercall_iret
>>> ENDPATCH(xen_iret)
>>>
>>> This seems rather unlikely to work on the espfix stack.
>>>
>>> Maybe espfix64 should be disabled when running on Xen and Xen should
>>> implement its own espfix64 in the hypervisor.
>>
>> Perhaps the first question is: is espfix even necessary on Xen?  How
>> does the Xen PV IRET handle returning to a 16-bit stack segment?
>>
>
> Test case here:
>
> https://gitorious.org/linux-test-utils/linux-clock-tests/source/dbfe196a0f6efedc119deb1cdbb0139dbdf609ee:
>
> It's sigreturn_32 and sigreturn_64.  Summary:
>
> (sigreturn_64 always fails unless my SS patch is applied.  results
> below for sigreturn_64 assume the patch is applied.  This is on KVM
> (-cpu host) on Sandy Bridge.)
>
> On Xen with espfix, both OOPS intermittently.
>
> On espfix-less kernels (Xen and non-Xen), 16-bit CS w/ 16-bit SS
> always fails.  Native (32-bit or 64-bit, according to the binary) CS
> with 16-bit SS fails for sigreturn_32, but passes for sigreturn_64.  I
> find this somewhat odd.  Native ss always passes.
>
> So I think that Xen makes no difference here, aside from the bug.
>
> That being said, I don't know whether Linux can do espfix64 at all
> when Xen is running -- for all I know, the IRET hypercall switches
> stacks to a Xen stack.

Microcode is weird.  Without espfix:

[RUN]    64-bit CS (33), 32-bit SS (2b)
    SP: 8badf00d5aadc0de -> 8badf00d5aadc0de
[OK]    all registers okay
[RUN]    32-bit CS (23), 32-bit SS (2b)
    SP: 8badf00d5aadc0de -> 5aadc0de
[OK]    all registers okay
[RUN]    16-bit CS (7), 32-bit SS (2b)
    SP: 8badf00d5aadc0de -> 5aadc0de
[OK]    all registers okay
[RUN]    64-bit CS (33), 16-bit SS (f)
    SP: 8badf00d5aadc0de -> 8badf00d5aadc0de
[OK]    all registers okay
[RUN]    32-bit CS (23), 16-bit SS (f)
    SP: 8badf00d5aadc0de -> 5ae3c0de
[FAIL]    Reg 15 mismatch: requested 0x8badf00d5aadc0de; got 0x5ae3c0de
[RUN]    16-bit CS (7), 16-bit SS (f)
    SP: 8badf00d5aadc0de -> 5ae3c0de
[FAIL]    Reg 15 mismatch: requested 0x8badf00d5aadc0de; got 0x5ae3c0de

--Andy
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