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

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Date:   Sun, 20 May 2018 21:00:26 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] kernel: sys: fix potential Spectre v1



On 05/20/2018 07:50 PM, Gustavo A. R. Silva wrote:
> 
> 
> On 05/18/2018 03:44 PM, Gustavo A. R. Silva wrote:
>>>>
>>>> #ifndef sanitize_index_nospec
>>>> inline bool sanitize_index_nospec(unsigned long *index,
>>>>                                    unsigned long size)
>>>> {
>>>>          if (*index >= size)
>>>>                  return false;
>>>>          *index = array_index_nospec(*index, size);
>>>>
>>>>          return true;
>>>> }
>>>> #endif
>>>
>>> I think this is fine in concept, we already do something similar in
>>> mpls_label_ok(). Perhaps call it validate_index_nospec() since
>>> validation is something that can fail, but sanitization in theory is
>>> something that can always succeed.
>>>
>>
>> OK. I got it.
>>
>>> However, the problem is the data type of the index. I expect you would
>>> need to do this in a macro and use typeof() if you wanted this to be
>>> generally useful, and also watch out for multiple usage of a macro
>>> argument. Is it still worth it at that point?
>>>
>>
>> Yeah. I think it is worth it. I'll work on this during the weekend and 
>> send a proper patch for review.
>>
> 
> 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);

> +})
>   #endif /* _LINUX_NOSPEC_H */
> 
> Thanks
> -- 
> Gustavo