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

[Dave Hansen <dave.hansen@linux.intel.com>]

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.