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

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Date:   Thu, 15 Feb 2018 16:48:23 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Willy Tarreau <w@....eu>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode

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

> On 02/15/2018 04:25 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@...ux.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.