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

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Date:   Thu, 15 Feb 2018 16:51:38 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        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 ML <x86@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode

Andrew Cooper <andrew.cooper3@...rix.com> wrote:

> On 16/02/2018 00:25, 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.
> 
> Being 32bit is itself sufficient protection against Meltdown (as long as
> there nothing interesting of the kernels mapped below the 4G boundary).
> 
> However, a 32bit compatibility process try to attack with Spectre/SP2 to
> redirect speculation back into userspace, at which point (if successful)
> the pipeline will be speculating in 64bit mode, and Meltdown is back on
> the table.  SMEP will block this attack vector, irrespective of other
> SP2 defences the kernel may employ, but a fully SP2-defended kernel
> doesn't require SMEP to be safe in this case.

Based on Jann Horn’s description of the branch predictor, it basically only
holds the lowest 31-bits of the target address. There might be a subtle
problem if the prediction wrapsaround, but excluding this case, I do not see
how Spectre v2 can be used to jump into running user code.