Re: [Xen-devel] Crash with paravirt-ops 2.6.31.6 kernel

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 11/23/09 07:25, Ian Campbell wrote:
> On Sun, 2009-11-22 at 09:54 +0000, Bastian Blank wrote:
>   
>> On Tue, Nov 17, 2009 at 10:04:36PM +0300, William Pitcock wrote:
>>     
>>> [    1.254927] init[1] general protection ip:f779042f sp:ff9b0340 error:0
>>>       
>> Hmm, this looks like the old Debian bug 544145[1]. For some reason the
>> hypervisor jumps back into 64bit mode after a syscall instruction.
>> Workaround: vdso32=0 or deinstall libc6-i686,
>>     
> I just noticed that one of my test boxes has a AMD processor so I took a
> bit of a look into this.
>
> The issue seems to be with this bit of code in the hypervisor
> (xen/arch/x86/x86_64/entry.S):
>
>         restore_all_guest:
>                 ASSERT_INTERRUPTS_DISABLED
>                 RESTORE_ALL
>                 testw $TRAP_syscall,4(%rsp)
>                 jz    iret_exit_to_guest
>         
>                 addq  $8,%rsp
>                 popq  %rcx                    # RIP
>                 popq  %r11                    # CS
>                 cmpw  $FLAT_USER_CS32,%r11
>                 popq  %r11                    # RFLAGS
>                 popq  %rsp                    # RSP
>                 je    1f
>                 sysretq
>         1:      sysretl
>
> We are attempting to return to the Linux defined __USER_CS32 (0x23)
> which does not match the test for the Xen defined FLAT_USER_CS32
> (0xe023) and therefore we hit the sysretq instead of the sysretl which
> causes us to return with CS 0xe033 (FLAT_USER_CS64) instead of CS
> 0xe023.
>   

Ah, good detective work.

> This patch to the kernel fixes things but doesn't seem that
> satisfactory:
>   

It is a bit ugly.  I guess you could just assert that FLAT_USER_CS32 is
part of the iret ABI so the guest has to use it, which appears to be the
defacto definition.  The downside is that usermode could observe that it
has a non-standard cs selector; however, the Linux ABI doesn't define
the selector values (and they're different in native 32 bit vs compat
anyway, I think).

> diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
> index 02f496a..203586d 100644
> --- a/arch/x86/xen/xen-asm_64.S
> +++ b/arch/x86/xen/xen-asm_64.S
> @@ -93,7 +93,7 @@ ENTRY(xen_sysret32)
>       pushq $__USER32_DS
>       pushq PER_CPU_VAR(old_rsp)
>       pushq %r11
> -     pushq $__USER32_CS
> +     pushq $FLAT_USER_CS32
>       pushq %rcx
>  
>       pushq $VGCF_in_syscall
>
> Coming from the other angle we could fix this in the hypervisor by
> always returning to guest (user or kernel) via iret instead of sysret:
>
> diff -r e7a1eab70fac xen/arch/x86/x86_64/entry.S
> --- a/xen/arch/x86/x86_64/entry.S     Mon Nov 09 10:24:54 2009 +0000
> +++ b/xen/arch/x86/x86_64/entry.S     Mon Nov 23 15:15:39 2009 +0000
> @@ -48,22 +48,6 @@
>  restore_all_guest:
>          ASSERT_INTERRUPTS_DISABLED
>          RESTORE_ALL
> -        testw $TRAP_syscall,4(%rsp)
> -        jz    iret_exit_to_guest
> -
> -        addq  $8,%rsp
> -        popq  %rcx                    # RIP
> -        popq  %r11                    # CS
> -        cmpw  $FLAT_USER_CS32,%r11
> -        popq  %r11                    # RFLAGS
> -        popq  %rsp                    # RSP
> -        je    1f
> -        sysretq
> -1:      sysretl
> -
> -        ALIGN
> -/* No special register assumptions. */
> -iret_exit_to_guest:
>          addq  $8,%rsp
>  .Lft0:  iretq
>
> I think much of the issue stems from Xen defining several segment
> descriptors which are essentially equivalent to the ones Linux uses. It
> seems a bit ugly to expose these Xen defined descriptors to the guest
> when it hasn't explicitly asked for them. On the other hand I'm not sure
> what can realistically do since doing the Right Thing would seem to
> involve looking up the descriptor in the GDT to determine if the
> selector is 32 or 64 bit and/or context switching IA32_STAR in some
> fashion to allow guests to specify their own userspace CS for sysret 32
> and 64.
>   

That would be a bit awkward to do in the iret fast path.

> Perhaps simply not returning guest userspace with sysret (as above)
> makes most sense, a syscall already takes a trap through the hypervisor
> on both entry and exit so I'm not sure the difference between sysret and
> iret is going to be noticeable.
>
> Another option might be to define VGCF_compat_mode as a new flag to
> HYPERVISOR_iret and select sysretq/sysretl based on that. This would
> still expose Xen descriptors to guests which didn't ask for one but at
> least it would (probably) be a compatible descriptor.
>   

I don't think that's much of an improvement over using the Xen selector
for cs.  Of course, it requires that the Xen selectors are actually part
of the ABI and won't change at some later point.

    J

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