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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Thu, 15 Feb 2018 16:42:51 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Nadav Amit <nadav.amit@...il.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

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.