Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

Stas Sergeev
2017-03-10T21:37:19Z

11.03.2017 00:04, Andy Lutomirski пишет:
> On Fri, Mar 10, 2017 at 2:30 AM, Stas Sergeev <stsp@list.ru> wrote:
>> 10.03.2017 05:41, Andy Lutomirski пишет:
>>
>>> On Wed, Mar 8, 2017 at 5:11 PM, Ricardo Neri
>>> <ricardo.neri-calderon@linux.intel.com> wrote:
>>>> On Wed, 2017-03-08 at 19:53 +0300, Stas Sergeev wrote:
>>>>> 08.03.2017 19:46, Andy Lutomirski пишет:
>>>>>>> No no, since I meant prot mode, this is not what I need.
>>>>>>> I would never need to disable UMIP as to allow the
>>>>>>> prot mode apps to do SLDT. Instead it would be good
>>>>>>> to have an ability to provide a replacement for the dummy
>>>>>>> emulation that is currently being proposed for kernel.
>>>>>>> All is needed for this, is just to deliver a SIGSEGV.
>>>>>> That's what I meant. Turning off FIXUP_UMIP would leave UMIP on but
>>>>>> turn off the fixup, so you'd get a SIGSEGV indicating #GP (or a vm86
>>>>>> GP exit).
>>>>> But then I am confused with the word "compat" in
>>>>> your "COMPAT_MASK0_X86_UMIP_FIXUP" and
>>>>> "sys_adjust_compat_mask(int op, int word, u32 mask);"
>>>>>
>>>>> Leaving UMIP on and only disabling a fixup doesn't
>>>>> sound like a compat option to me. I would expect
>>>>> compat to disable it completely.
>>>> I guess that the _UMIP_FIXUP part makes it clear that emulation, not
>>>> UMIP is disabled, allowing the SIGSEGV be delivered to the user space
>>>> program.
>>>>
>>>> Would having a COMPAT_MASK0_X86_UMIP_FIXUP to disable emulation and a
>>>> COMPAT_MASK0_X86_UMIP to disable UMIP make sense?
>>>>
>>>> Also, wouldn't having a COMPAT_MASK0_X86_UMIP to disable UMIP defeat its
>>>> purpose? Applications could simply use this compat mask to bypass UMIP
>>>> and gain access to the instructions it protects.
>>>>
>>> I was obviously extremely unclear. The point of the proposed syscall
>>> is to let programs opt out of legacy features.
>> I guess both "compat" and "legacy" are misleading
>> here. Maybe these are "x86-specific" or "hypervisor-specific",
>> but a mere enabling of UMIP doesn't immediately make
>> the use of SLDT instruction a legacy IMHO.
> Sure it is. :) Using SLDT from user mode is a legacy ability that
> just happens to still work on existing CPUs and kernels. Once UMIP
> goes in, it will officially be obsolete
Yes, but the names you suggest, imply that "UMIP_FIXUP"
is legacy or compat, which I find misleading because it have
just appeared. Maybe something like "COMPAT_X86_UMIP_INSNS_EMU"?