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

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Message-ID: <9521b7db-0ff5-21db-f744-b818cd640783@embeddedor.com>
Date:   Fri, 18 May 2018 17:11:53 -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 05:08 PM, Dan Williams wrote:
>>
>> Oh I see now. Just to double check, then something like the following would
>> be broken too, because is basically the same as the code above, and well, it
>> doesn't make much sense to store the value returned by macro
>> array_index_nospec into x, correct?:
> 
> Correct, broken:
> 
>>
>> bool foo(int x)
>> {
>>          if(x >= MAX)
>>                  return false;
> 
> Under speculation we may not return here when x is greater than max.
> 
>>          x = array_index_nospec(x, MAX);
> 
> x is now sanitized under speculation to zero, but the compiler would
> likely just throw this away because nothing consumes it.
> 
>>          return true;
>> }
>>
>> int vulnerable(int x)
>> {
>>          if(!foo(x))
>>                  return -1;
> 
> cpu might speculate that this branch is not taken...
> 
>>
>>          temp = array[x];
> 
> ...so x had better be bounded here, otherwise Spectre.
> 

I got it.

I appreciate the feedback.

Thanks, Dan.
--
Gustavo