Re: [PATCH v1 2/8] s390/sclp: check sccb len before filling in data

[Janosch Frank <frankja@linux.ibm.com>]

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On 5/12/20 6:01 PM, Cornelia Huck wrote:
> On Mon, 11 May 2020 17:02:06 +0200
> David Hildenbrand <david@redhat.com> wrote:
> 
>> On 11.05.20 16:50, Janosch Frank wrote:
>>> On 5/11/20 4:44 PM, David Hildenbrand wrote:  
>>>> On 11.05.20 16:36, Janosch Frank wrote:  
>>>>> On 5/9/20 1:08 AM, Collin Walling wrote:  
>>>>>> The SCCB must be checked for a sufficient length before it is filled
>>>>>> with any data. If the length is insufficient, then the SCLP command
>>>>>> is suppressed and the proper response code is set in the SCCB header.
>>>>>>
>>>>>> Signed-off-by: Collin Walling <walling@linux.ibm.com>  
>>>>>
>>>>> Fixes tag?
> 
> Probably
> 
> Fixes: 832be0d8a3bb ("s390x: sclp: Report insufficient SCCB length")
> 
> ?
> 
>>>>> Reviewed-by: Janosch Frank <frankja@linux.ibm.com>  
>>>>
>>>> This is not a fix AFAIKs.
>>>> sclp_service_call()/sclp_service_call_protected() always supplies a full
>>>> SCCB of exactly 4k size.
>>>>  
>>>
>>> We don't check for QEMU's 4k buffer here, but for the length that was
>>> specified by the guest.
>>>
>>> It's valid for the guest to request cpu info and state that its buffer
>>> is only 1k. We can't write everything in 1k if we have ~200 cpus, so
>>> we'll report the insufficient length rc.
>>>
>>> What he fixes here is the time of the length check, it should be done
>>> before any changes are being done to the work_sccb.  
>>
>> I don't have access to the spec, especially, if the guest can expect
>> nothing else in the sccb to change in case we report an error code. So
>> whatever you tell me, I have to trust you :)
> 
> Same here. Sounds plausible, but I have to trust the folks with the
> documentation :)
> 

The AR states that:
* Command validity check (has prio over length, as length is dependent
on command)
* boundary (if extended-length is not available)
* Sufficient length check

are done before "any other command action is taken".
If a test fails the command is suppressed.





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