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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/04/17 10:46, Jan Beulich wrote:
>>>> On 21.04.17 at 11:11, <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxxxx> wrote:
>>>>> From: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
>>>> 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@xxxxxxxxxx>
>>>>> Signed-off-by: Jennifer Herbert <Jennifer.Herbert@xxxxxxxxxx>
>>>> ... 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?

If the caller doesn't provide all the information the hypervisor half of
the interface expects, it should get a hard error, not a truncated
attempt to fulful the request.

The existing behaviour is simply broken.

~Andrew

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