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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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?

Considering this is a controversial change, I think it is a bad idea
to merge this into the otherwise uncontroversial change here.

Jan


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