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

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Message-ID: <f6be31b7-a6f8-3d7f-f621-09490a1b0489@embeddedor.com>
Date:   Fri, 18 May 2018 16:27:24 -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 05/18/2018 03:44 PM, Gustavo A. R. Silva wrote:
>>>>
>>>
>>> Oops, it seems I sent the wrong patch. The function would look like 
>>> this:
>>>
>>> #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.
> 
> Thanks for the feedback.

BTW, I'm analyzing other cases, like the following:

bool foo(int x)
{
          if(!validate_index_nospec(&x))
                  return false;

          [...]

          return true;
}

int vulnerable(int x)
{
          if (!foo(x))
                  return -1;

          temp = array[x];

          [...]

}

Basically my doubt is how deep this barrier can be placed into the call 
chain in order to continue working.

Thanks
--
Gustavo