Re: [Xen-devel] [PATCH] VMX: don't crash processing 'd' debug key

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/11/13 16:04, Jan Beulich wrote:
>>>> On 07.11.13 at 20:08, Tim Deegan <tim@xxxxxxx> wrote:
>> At 10:44 +0000 on 07 Nov (1383817496), Jan Beulich wrote:
>>> @@ -675,7 +675,17 @@ void vmx_get_segment_register(struct vcp
>>>  {
>>>      unsigned long attr = 0, sel = 0, limit;
>>>  
>>> -    vmx_vmcs_enter(v);
>>> +    /*
>>> +     * We may get here in the context of dump_execstate(), which may have
>>> +     * interrupted context switching between setting "current" and
>>> +     * vmx_do_resume() reaching the end of vmx_load_vmcs(). That would make
>>> +     * all the VMREADs below fail if we don't bail right away.
>>> +     */
>>> +    if ( unlikely(!vmx_vmcs_enter(v)) )
>>> +    {
>>> +        memset(reg, 0, sizeof(*reg));
>>> +        return;
>> It would be nice to print something here, at least on the first
>> instance.  Otherwise someone looking at bizarre debugkey output would
>> have to know (and remember) about this path.
> Did this.
>
>> I'd also be inclined to ASSERT that, e.g. interrupts are disabled here
>> -- if for any reason this function ever starts corrupting register
>> state on other paths, we'll want to know about it quickly!
> But I'm rather hesitant to do this. If anything, we'd need per-CPU
> state tracking whether we're in do_invalid_op()'s main switch.
>
> Jan
>

I agree - the debug keys are hardly normal operation, and we don't want
to ASSERT() in a debugkey.

Perhaps an alternative would be a short printk indicating that if this
is debugkey then the caller was unlucky and should try again, as we know
there is a short vulnerable window?

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