Re: [PATCH v2 2/3] xen/domain: fix UBSAN null pointer dereference of d->shared_info

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.06.2026 13:57, Oleksii Kurochko wrote:
> 
> 
> On 6/3/26 1:23 PM, Jan Beulich wrote:
>> On 03.06.2026 13:05, Oleksii Kurochko wrote:
>>>
>>>
>>> On 6/3/26 7:54 AM, Jan Beulich wrote:
>>>>> --- a/xen/common/event_fifo.c
>>>>> +++ b/xen/common/event_fifo.c
>>>>> @@ -562,9 +562,10 @@ static void setup_ports(struct domain *d, unsigned
>>>>> int prev_evtchns)
>>>>>
>>>>>             evtchn = evtchn_from_port(d, port);
>>>>>
>>>>> -        if ( d->shared_info &&
>>>>> -             guest_test_bit(d, port, &shared_info(d, evtchn_pending)) )
>>>>> +#ifdef CONFIG_HAS_SHARED_INFO
>>>>> +        if ( guest_test_bit(d, port, &shared_info(d, evtchn_pending)) )
>>>>>                 evtchn->pending = true;
>>>>> +#endif
>>>> While as per above shared_info() would best not exist when !HAS_SHARED_INFO
>>>> (in which case #ifdef may be unavoidable here), an alternative where
>>>> IS_ENABLED() could be used here may want at least considering. E.g.
>>>> causing a link-time failure when shared_info() is used (and not compiled
>>>> out).
>>>
>>> We still want here to have #ifdef instead of IS_ENABLED() as
>>> shared_info() shouldn't exist for arch without 2L support so it will end
>>> with linkage error.
>>
>> I don't understand this part.
> 
> If the change will look like:
> 
>    if ( IS_ENABLED(CONFIG_HAS_SHARED_INFO) && guest_test_bit(d, port, 
> &shared_info(d, evtchn_pending)) )
>        evtchn->pending = true;
> 
> It will help to avoid NULL pointer dereference of shared info page in 
> case of 2L isn't supported. But considering that shared_info() macros 
> will be reworked in the way which will lead to linkage error in the case 
> when it is used and arch doesn't have shared info page support usage of 
> shared_info() in setup_ports() will lead to linkage error what is wanted 
> to be avoid for arch without 2L support.

What linking error are you talking of? When !HAS_SHARED_INFO, the compiler
will DCE the entire guest_test_bit(), including the shared_info() use. IOW
I still don't understand what you're talking of.

Jan