Re: [PATCH v3 2/4] x86, mpx: hook #BR exception handler to allocate bound tables

[Andy Lutomirski <luto@amacapital.net>]

On Mon, Jan 27, 2014 at 7:35 PM, Ren Qiaowei <qiaowei.ren@intel.com> wrote:
> On 01/28/2014 04:36 AM, Andy Lutomirski wrote:
>>>
>>> +       bd_entry = status & MPX_BNDSTA_ADDR_MASK;
>>> +       if ((bd_entry >= bd_base) && (bd_entry < bd_base + bd_size))
>>> +               allocate_bt(bd_entry);
>>
>>
>> What happens if this fails?  Retrying forever isn't very nice.
>>
> If allocation of the bound table fail, the related entry in the bound
> directory is still invalid. The following access to this entry still produce
> #BR fault.
>

By the "following access" I think you mean the same instruction that
just trapped -- it will trap again because the exception hasn't been
fixed up.  Then mmap will fail again, and you'll retry again, leading
to an infinite loop.

I think that failure to fix up the exception should either let the
normal bounds error through or should raise SIGBUS.

>
>>> +       if (!user_mode(regs)) {
>>> +               if (!fixup_exception(regs)) {
>>> +                       tsk->thread.error_code = error_code;
>>> +                       tsk->thread.trap_nr = X86_TRAP_BR;
>>> +                       die("bounds", regs, error_code);
>>> +               }
>>
>>
>> Why the fixup?  Unless I'm missing something, the kernel has no business
>> getting #BR on access to a user address.
>>
>> Or are you adding code to allow the kernel to use MPX itself?  If so,
>> shouldn't this use an MPX-specific fixup to allow normal C code to use
>> this stuff?
>>
> It checks whether #BR come from user-space. You can see do_trap_no_signal().

Wasn't #BR using do_trap before?  do_trap doesn't call
fixup_exception.  I don't see why it should do it now.  (I also don't
think it should come from kernel space until someone adds kernel-mode
MPX support.)

>
>
>>> +               goto exit;
>>> +       }
>>> +
>>> +       if (!boot_cpu_has(X86_FEATURE_MPX)) {
>>> +               do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code,
>>> NULL);
>>> +               goto exit;
>>
>>
>> This, as well as the status == 0 case, should probably document that the
>> exception is from BOUND, not MPX.
>>
> Ok. I will add one comment for this.
>
>
>
>>> +               break;
>>> +
>>> +       case 1: /* Bound violation. */
>>> +       case 0: /* No MPX exception. */
>>> +               do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code,
>>> NULL);
>>> +               break;
>>
>>
>> What does "No Intel MPX exception" mean?  Surely that has business
>> sending #BR.
>>
> Oh. It comes from spec, and just mean it is not from MPX. :) I will change
> it to be accurate.
>
>
>>> +
>>> +       default:
>>> +               break;
>>
>>
>> What does status 3 mean?  The docs say "reserved".  Presumably this
>> should log and kill the process.
>
> I guess it should be a good suggestion.
>
> Thanks,
> Qiaowei
>



-- 
Andy Lutomirski
AMA Capital Management, LLC