Re: [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 07/27/2015 11:53 AM, Andy Lutomirski wrote:
> On Mon, Jul 27, 2015 at 8:36 AM, Boris Ostrovsky
> <boris.ostrovsky@oracle.com> wrote:
>> On 07/25/2015 01:36 AM, Andy Lutomirski wrote:
>>> Here's v3.  It fixes the "dazed and confused" issue, I hope.  It's also
>>> probably a good general attack surface reduction, and it replaces some
>>> scary code with IMO less scary code.
>>>
>>> Also, servers and embedded systems should probably turn off modify_ldt.
>>> This makes that possible.
>>>
>>> Xen people, can you take a look at this?
>>>
>>> Willy and Kees: I left the config option alone.  The -tiny people will
>>> like it, and we can always add a sysctl of some sort later.
>>>
>>> Changes from v3:
>>>    - Hopefully fixed Xen.
>>
>> 32b-on-32b fails in the same manner. (but non-zero LDT is taken care of)
>>
>>>    - Fixed 32-bit test case on 32-bit native kernel.
>>
>> I am not sure I see what changed.
> I misplaced the fix in the wrong git commit, so I failed to sent it.  Oops.
>
> I just sent v4.1 of patch 3.  Can you try that?


I am hitting BUG() in Xen code (returning from a hypercall) when freeing 
LDT in destroy_context(). Interestingly though when I run the test in 
the debugger I get SIGILL (just like before) but no BUG().

Let me get back to you on that later today.


-boris