Re: [PATCH] kernel: sys: fix potential Spectre v1

[Dan Williams]

On Tue, May 22, 2018 at 10:03 PM, Gustavo A. R. Silva
<gustavo@xxxxxxxxxxxxxx> wrote:
>
>
> On 05/22/2018 03:50 PM, Dan Williams wrote:
>>>>
>>>>
>>>> Dan,
>>>>
>>>> What do you think about this first draft:
>>>>
>>>> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
>>>> index e791ebc..6154183 100644
>>>> --- a/include/linux/nospec.h
>>>> +++ b/include/linux/nospec.h
>>>> @@ -55,4 +55,16 @@ static inline unsigned long
>>>> array_index_mask_nospec(unsigned long index,
>>>>
>>>> \
>>>>           (typeof(_i)) (_i & _mask);
>>>> \
>>>>    })
>>>> +
>>>> +#define validate_index_nospec(index, size)                            \
>>>> +({                                                                    \
>>>> +       typeof(index) *ptr = &(index);                                 \
>>>> +       typeof(size) _s = (size);                                      \
>>>> +                                                                      \
>>>> +       BUILD_BUG_ON(sizeof(*ptr) > sizeof(long));                     \
>>>> +       BUILD_BUG_ON(sizeof(_s) > sizeof(long));                       \
>>>> +                                                                      \
>>>> +       *ptr >= _s ? false :                                           \
>>>> +       (*ptr = array_index_nospec(*ptr, _s) ? true : true);           \
>>>
>>>
>>>
>>> This actually should be:
>>>
>>> ((*ptr = array_index_nospec(*ptr, _s)) ? true : true);
>>>
>>
>> Let's not use ternary conditionals at all to make this more readable.
>>
>
> OK. How about this:
>
> diff --git a/include/linux/nospec.h b/include/linux/nospec.h
> index e791ebc..498995b 100644
> --- a/include/linux/nospec.h
> +++ b/include/linux/nospec.h
> @@ -55,4 +55,21 @@ static inline unsigned long
> array_index_mask_nospec(unsigned long index,
>                                                                        \
>         (typeof(_i)) (_i & _mask);                                     \
>  })
> +
> +#define validate_index_nospec(index, size)                            \
> +({                                                                    \
> +       bool ret = true;                                               \
> +       typeof(index) *ptr = &(index);                                 \
> +       typeof(size) _s = (size);                                      \
> +                                                                      \
> +       BUILD_BUG_ON(sizeof(*ptr) > sizeof(long));                     \
> +       BUILD_BUG_ON(sizeof(_s) > sizeof(long));                       \
> +                                                                      \
> +       if (*ptr >= size)                                              \
> +               ret = false;                                           \
> +                                                                      \
> +       *ptr = array_index_nospec(*ptr, _s);                           \
> +                                                                      \
> +       ret;                                                           \
>
> +})
>  #endif /* _LINUX_NOSPEC_H */

Assuming the assembly generation is comparable with the open coded
version, this looks ok to me.