Re: [PATCH 2/4] hvm/dmop: Implement copy_{to, from}_guest_buf() in terms of raw accessors

[Jennifer Herbert <Jennifer.Herbert@citrix.com>]

On 21/04/17 11:38, Jan Beulich wrote:
>>>> On 21.04.17 at 12:11, <Jennifer.Herbert@citrix.com> wrote:
>> On 21/04/17 10:46, Jan Beulich wrote:
>>>>>> On 21.04.17 at 11:11, <andrew.cooper3@citrix.com> wrote:
>>>> On 21/04/2017 09:54, Andrew Cooper wrote:
>>>>> On 21/04/2017 08:27, Jan Beulich wrote:
>>>>>>>>> On 20.04.17 at 19:59, <jennifer.herbert@citrix.com> wrote:
>>>>>>> From: Jennifer Herbert <Jennifer.Herbert@citrix.com>
>>>>>> Is this correct, considering that iirc the patch was new in v5 and ...
>>>>>>
>>>>>>> This also allows the usual cases to be simplified, by omitting an
>>>>>>> unnecessary
>>>>>>> buf parameters, and because the macros can appropriately size the object.
>>>>>>>
>>>>>>> This makes copying to or from a buf that isn't big enough an error.
>>>>>>> If the buffer isnt big enough, trying to carry on regardless
>>>>>>> can only cause trouble later on.
>>>>>>>
>>>>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>>>>>>> Signed-off-by: Jennifer Herbert <Jennifer.Herbert@citrix.com>
>>>>>> ... this sequence of S-o-b-s?
>>>>>>
>>>>>>> --- a/xen/arch/x86/hvm/dm.c
>>>>>>> +++ b/xen/arch/x86/hvm/dm.c
>>>>>>> @@ -32,36 +32,47 @@ struct dmop_args {
>>>>>>>        struct xen_dm_op_buf buf[2];
>>>>>>>    };
>>>>>>>    
>>>>>>> -static bool copy_buf_from_guest(const xen_dm_op_buf_t bufs[],
>>>>>>> -                                unsigned int nr_bufs, void *dst,
>>>>>>> -                                unsigned int idx, size_t dst_size)
>>>>>>> +static bool _raw_copy_from_guest_buf(void *dst,
>>>>>>> +                                     const struct dmop_args *args,
>>>>>>> +                                     unsigned int buf_idx,
>>>>>>> +                                     size_t dst_bytes)
>>>>>>>    {
>>>>>>> -    size_t size;
>>>>>>> +    size_t buf_bytes;
>>>>>>>    
>>>>>>> -    if ( idx >= nr_bufs )
>>>>>>> +    if ( buf_idx >= args->nr_bufs )
>>>>>>>            return false;
>>>>>>>    
>>>>>>> -    memset(dst, 0, dst_size);
>>>>>>> +    buf_bytes =  args->buf[buf_idx].size;
>>>>>>>    
>>>>>>> -    size = min_t(size_t, dst_size, bufs[idx].size);
>>>>>>> +    if ( dst_bytes > buf_bytes )
>>>>>>> +        return false;
>>>>>> While this behavioral change is now being mentioned in the
>>>>>> description, I'm not sure I buy the argument of basically being
>>>>>> guaranteed to cause trouble down the road. Did you consider the
>>>>>> forward compatibility aspect here, allowing us to extend interface
>>>>>> structures by adding fields to their ends without breaking old
>>>>>> callers? Paul, what are your thoughts here?
>>>>> DMOP is a stable ABI.  There is no legal extending of any objects.
>>>>>
>>>>> The previous semantics are guaranteed to break the ABI with future
>>>>> subops, which is why I removed it.  In the case that the guest
>>>>> originally passed an overly long buffer, and someone tried be "clever"
>>>>> here passing the same object here expecting _raw_copy_from_guest_buf()
>>>>> to DTRT, the function will end up copying too much data from the guest,
>>>>> and you will end up with something the guest wasn't intending to be part
>>>>> of the structure replacing the zero extension.
>>>>>
>>>>> New subops which take a longer object should use a brand new object.
>>>>>
>>>>> $MAGIC compatibility logic like you want has no business living in the
>>>>> copy helper.  Had I spotted the intention during the original dmop
>>>>> series, I would have rejected it during review.
>>>> Actually, the existing behaviour is already broken.  If a guest passes
>>>> an overly short buf[0], the dmop logic won't get a failure, and instead
>>>> get a truncated structure which has been zero extended.
>>>>
>>>> This is very definitely the wrong thing to do, because such a truncated
>>>> structure might actually contain some legitimate operations.
>>> So what, if that's what the caller meant to happen?
>>>
>>> Considering this is a controversial change, I think it is a bad idea
>>> to merge this into the otherwise uncontroversial change here.
>> How about we move the min(..) and memset out of the helper function, and
>> into dm_op?
> Wouldn't that result in different buffers being treated differently,
> which I'd rather not see happening?

I think in the general case, its wrong to assume that its ok to truncate 
a buffer.
In specific cases, such as buf[0], we don't really know how big the buf 
needs to be until we start parsing it. In this case, we want to say 
'give me what you have, upto this maximum'.  This is the special case, 
not the general one, an as such does not belong in the general case 
accesser helper.
Certainly when we get to updating the accesser helper to deal with 
offsets, its get more confused.  If we give it an offset greater then 
the size of the buffer, do we just memset?  That would seem wrong.  With 
an offset of 0, we want it to do the memset thing, to deal with buf[0].  
But what about if we have an offset.  I'd think the calling function 
would really want to know if its got nonsense.

Another solution would be to have two variants of the helper function. A 
'normal' one, and the best effort one.

-jenny


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