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Re: [Xen-devel] [semi-urgent Xen CS question] Re: git commit 9fd67b4ed07

To: Andrew Lutomirski <luto@xxxxxxx>,Keir Fraser <keir.xen@xxxxxxxxx>
Subject: Re: [Xen-devel] [semi-urgent Xen CS question] Re: git commit 9fd67b4ed0714ab718f1f9bd14c344af336a6df7 (x86-64: Give vvars their own page) breaks Xen PV guests (64-bit).
From: "j.fitz.inge@xxxxxxxxx" <jeremy@xxxxxxxx>
Date: Tue, 26 Jul 2011 16:37:33 -0700
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
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The correct fix is to just look at the cpl in cs and ignore the rest of the selector.

Sent from my Android phone with K-9 Mail. Please excuse my brevity.

Andrew Lutomirski <luto@xxxxxxx> wrote:
On Tue, Jul 26, 2011 at 5:10 PM, Andrew Lutomirski <luto@xxxxxxx> wrote:
> On Tue, Jul 26, 2011 at 4:48 PM, Keir Fraser <keir.xen@xxxxxxxxx> wrote:
>> On 26/07/2011 20:08, "Andrew Lutomirski" <luto@xxxxxxx> wrote:
>>> On Tue, Jul 26, 2011 at 11:32 AM, Konrad Rzeszutek Wilk
>>> <konrad.wilk@xxxxxxxxxx> wrote:
>>>> On Mon, Jul 25, 2011 at 09:50:30PM -0400, Andrew Lutomirski wrote:
>>>>> After staring at the Xen assembly code with vague comprehension, I
>>>>> think I can sort of understand what's going on.
>>>> Ok.
>>>>> Can you run this little program on a working kernel and tell me what
>>>>> it says (built as 64-bit and as 32-bit (with -m32)):
>>>> 32-bit:
>>>> [konrad@f13-x86-build ~]$ ./check
>>>> cs = 73
>>>> [konrad@f13-x86-build ~]$ uname -a
>>>> Linux f13-x86-build.dumpdata.com 3.0.0 #1 SMP PREEMPT Tue Jul 26 09:56:38 EDT
>>>> 2011 i686 i686 i386 GNU/Linux
>>>> 64-bit:
>>>> [konrad@f13-amd64-build ~]$ ./check
>>>> cs = e033
>>> My best guess is that each task starts out with standard __USER_CS,
>>> but the code in write_stack_trampoline (in the hypervisor) tells the
>>> kernel that CS is 0xe033 and then the next return to userspace makes
>>> it true.
>> Yes, that's right.
> But it's still weird, because AFAICT xen_sysret64 already does the
> right thing.  So presumably the failure case only happens when
> something prevents sysret from working, like CONFIG_AUDITSYSCALL.

I lied. I still don't see what's going on.

Xen, in enlighten.c, registers xen_syscall_target as the 64-bit
syscall target (or at least I assume that's what CALLBACKTYPE_syscall

xen_syscall_target does this:

.macro undo_xen_syscall
mov 0*8(%rsp), %rcx
mov 1*8(%rsp), %r11
mov 5*8(%rsp), %rsp

/* Normal 64-bit system call target */
jmp system_call_after_swapgs

So the 0xe033 that Xen writes is popped back off the kernel stack and ignored.

xen_sysret64 explicitly pushes __USER_CS as its CS value, so that path looks OK.

If we go into the iret patch (via auditing, for example), then the
FIXUP_TOP_OF_STACK macro does movq $__USER_CS,CS+\offset(%rsp), which
(unless it's buggy) writes __USER_CS into the appropriate spot.

So I don't see what part of the entry path needs patching.


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