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

[Masami Hiramatsu]

(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.

Thank you,

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


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