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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.11.13 at 17:09, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> 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.
> 
> 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?

Hence the addition of a one time printk() as Tim suggested.

Jan


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