Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode

Nadav Amit
2018-02-16T00:48:28Z

Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 02/15/2018 04:25 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>
>>> On 02/15/2018 08:35 AM, Nadav Amit wrote:
>>>> I removed the PTI disabling while SMEP is unsupported, although I
>>>> must admit I did not fully understand why it is required.
>>>
>>> Do you mean you don't fully understand how PTI gives SMEP-like behavior
>>> on non-SMEP hardware?
>>
>> No. I understand how it provide SMEP-like behavior, and I understand the value
>> of SMEP by itself.
>>
>> However, I do not understand why SMEP-like protection is required to protect
>> processes that run in compatibility-mode from Meltdown/Spectre attacks. As
>> far as I understand, the process should not be able to manipulate the kernel
>> to execute code in the low 4GB.
>
> There are two problems: one is that regardless of Meltdown/Spectre, SMEP
> is valuable. It's valuable to everything, compatibility-mode or not.
>
> The second problem is the RSB. It has a full-width virtual address and,
> unlike the other indirect branch prediction, can steer you anywhere
> including to the low 4GB.

Thanks for the explanation. Based on Linus response, I guess this series is
nak’d, but still thanks for your patience.

I suspected the RSB might be the reason but it seemed to me that all the ROP
opportunities are still there, so I assumed it is not a reason.

Anyhow, thanks again.