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SubjectRe: [PATCH] kernel: sys: fix potential Spectre v1
On Tue, May 22, 2018 at 10:03 PM, Gustavo A. R. Silva
<gustavo@embeddedor.com> 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.

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