linux-kernel - Re: x2apic_wrmsr_fence vs. Intel manual

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]

Message-ID: <602968e1-c1e4-0980-effa-e9c40b82c8c8@intel.com>
Date:   Wed, 4 Mar 2020 10:27:08 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jan Kiszka <jan.kiszka@...mens.com>, x86 <x86@...nel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x2apic_wrmsr_fence vs. Intel manual

On 3/2/20 8:11 AM, Jan Kiszka wrote:
> The former dates back to ce4e240c279a, but that commit does not mention 
> why lfence is not needed. Did the manual read differently back then? Or 
> why are we safe? To my reading of lfence, it also has a certain 
> instruction serializing effect that mfence does not have.

I asked around Intel about this.

The old "SFENCE, or MFENCE" recommendation was deemed insufficient
because it has no impact on the ordering of WRMSR since it is not a
"load or store instruction".  LFENCE's instruction-ordering semantic is
needed because it ensures later ordering of all instructions, not just
loads and stores.

Jan, do you think you're seeing a bug resulting from WRMSR ordering?