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

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Date:   Fri, 18 May 2018 15:08:09 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.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 Fri, May 18, 2018 at 3:01 PM, Gustavo A. R. Silva
<gustavo@...eddedor.com> wrote:
>
>
> On 05/18/2018 04:45 PM, Dan Williams wrote:
>>
>> On Fri, May 18, 2018 at 2:27 PM, Gustavo A. R. Silva
>> <gustavo@...eddedor.com> wrote:
>>>
>>>
>>>
>>> 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.
>>
>>
>> This is broken you would need to pass the address of x into foo()
>> otherwise there may be speculation on the return value of foo.
>>
>
> 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.