linux-kernel - Re: [PATCH v2] x86/xen/64: Rearrange the SYSCALL entries

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Date:   Mon, 14 Aug 2017 08:46:00 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andrew Cooper <andrew.cooper3@...rix.com>, X86 ML <x86@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2] x86/xen/64: Rearrange the SYSCALL entries

On Mon, Aug 14, 2017 at 1:53 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Sun, Aug 13, 2017 at 7:44 PM, Brian Gerst <brgerst@...il.com> wrote:
>> On Mon, Aug 7, 2017 at 11:59 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>>  /* Normal 64-bit system call target */
>>>  ENTRY(xen_syscall_target)
>>> -       undo_xen_syscall
>>> -       jmp entry_SYSCALL_64_after_swapgs
>>> +       popq %rcx
>>> +       popq %r11
>>> +       jmp entry_SYSCALL_64_after_hwframe
>>>  ENDPROC(xen_syscall_target)
>>>
>>>  #ifdef CONFIG_IA32_EMULATION
>>>
>>>  /* 32-bit compat syscall target */
>>>  ENTRY(xen_syscall32_target)
>>> -       undo_xen_syscall
>>> -       jmp entry_SYSCALL_compat
>>> +       popq %rcx
>>> +       popq %r11
>>> +       jmp entry_SYSCALL_compat_after_hwframe
>>>  ENDPROC(xen_syscall32_target)
>>>
>>>  /* 32-bit compat sysenter target */
>>>  ENTRY(xen_sysenter_target)
>>> -       undo_xen_syscall
>>> +       mov 0*8(%rsp), %rcx
>>> +       mov 1*8(%rsp), %r11
>>> +       mov 5*8(%rsp), %rsp
>>>         jmp entry_SYSENTER_compat
>>>  ENDPROC(xen_sysenter_target)
>>
>> This patch causes the iopl_32 and ioperm_32 self-tests to fail on a
>> 64-bit PV kernel.  The 64-bit versions pass. It gets a seg fault after
>> "parent: write to 0x80 (should fail)", and the fault isn't caught by
>> the signal handler.  It just dumps back to the shell.  The tests pass
>> after reverting this.
>
> I can reproduce it if I emulate an AMD machine.  I can "fix" it like this:

Yes, this is an AMD processor.

> diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
> index a8a4f4c460a6..6255e00f425e 100644
> --- a/arch/x86/xen/xen-asm_64.S
> +++ b/arch/x86/xen/xen-asm_64.S
> @@ -97,6 +97,9 @@ ENDPROC(xen_syscall_target)
>  ENTRY(xen_syscall32_target)
>         popq %rcx
>         popq %r11
> +       movq $__USER32_DS, 4*8(%rsp)
> +       movq $__USER32_CS, 1*8(%rsp)
> +       movq %r11, 2*8(%rsp)
>         jmp entry_SYSCALL_compat_after_hwframe
>  ENDPROC(xen_syscall32_target)
>
> but I haven't tried to diagnose precisely what's going on.
>
> Xen seems to be putting the 0xe0?? values in ss and cs, which oughtn't
> to be a problem, but it kills opportunistic sysretl.  Maybe that's
> triggering a preexisting bug?

Resetting the CS/SS values worked.  Looking at the Xen hypervisor
code, EFLAGS on the stack should already be set to the value in R11,
so that part doesn't appear necessary.

Shouldn't this also be done for the 64-bit SYSCALL entry, for
consistency with previous code?

--
Brian Gerst