Re: [PATCH 3.19 v4 2/2] x86: Enforce maximum instruction size in the instruction decoder

[Andy Lutomirski]

On Thu, Jan 15, 2015 at 5:02 PM, Masami Hiramatsu
<masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> (2015/01/16 0:22), Andy Lutomirski wrote:
>> On Jan 15, 2015 4:37 AM, "Masami Hiramatsu"
>> <masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
>>>
>>> (2015/01/14 6:49), Andy Lutomirski wrote:
>>>> x86 instructions cannot exceed 15 bytes, and the instruction decoder
>>>> should enforce that.  Prior to 6ba48ff46f76, the instruction length
>>>> limit was implicitly set to 16, which was an approximation of 15,
>>>> but there is currently no limit at all.
>>>>
>>>> Fix the decoder to reject instructions that exceed 15 bytes.
>>>> A subsequent patch (targetted for 3.20) will fix MAX_INSN_SIZE.
>>>
>>> Hmm, is there any problem to just change MAX_INSN_SIZE to 15?
>>
>> I don't want to do that for 3.19.  It's kind of late.
>>
>>>
>>>> Other than potentially confusing some of the decoder sanity checks,
>>>> I'm not aware of any actual problems that omitting this check would
>>>> cause.
>>>>
>>>> Fixes: 6ba48ff46f76 x86: Remove arbitrary instruction size limit in instruction decoder
>>>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>>>> ---
>>>>  arch/x86/lib/insn.c | 7 +++++++
>>>>  1 file changed, 7 insertions(+)
>>>>
>>>> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
>>>> index 2480978b31cc..7b80745d2c5a 100644
>>>> --- a/arch/x86/lib/insn.c
>>>> +++ b/arch/x86/lib/insn.c
>>>> @@ -52,6 +52,13 @@
>>>>   */
>>>>  void insn_init(struct insn *insn, const void *kaddr, int buf_len, int x86_64)
>>>>  {
>>>> +     /*
>>>> +      * Instructions longer than 15 bytes are invalid even if the
>>>> +      * input buffer is long enough to hold them.
>>>> +      */
>>>> +     if (buf_len > 15)
>>>> +             buf_len = 15;
>>>> +
>>>
>>> Without changing the MAX_INSN_SIZE, this looks very odd, since all other
>>> code suppose that the max length of an instruction is 16 (MAX_INSN_SIZE)
>>> except here.
>>
>> I thought this was your suggestion.  Did I misunderstand?
>
> Yes, what I meant about "15" was the the "15" in the comment.
> So
>
> +     /*
> +      * Instructions longer than MAX_INSN_SIZE bytes are invalid even if the
> +      * input buffer is long enough to hold them.
> +      */
> +     if (buf_len > MAX_INSN_SIZE)
> +             buf_len = MAX_INSN_SIZE;
>
> is acceptable.
>
>> If you think the current code is okay for 3.19, I can fold the two
>> patches together and send for 3.20.
>
> If it does really cause a bug or a real problem, it must fix asap.
> If not, I'd like to fix this issue with changing MAX_INSN_SIZE to 15.
>

Since this has waited for quite a while and there's no known urgent
problem, I'll just send a combined patch.

--Andy

> Thank you,
>
> --
> Masami HIRAMATSU
> Software Platform Research Dept. Linux Technology Research Center
> Hitachi, Ltd., Yokohama Research Laboratory
> E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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