linux-kernel - Re: [PATCH] proc: Provide details on indirect branch speculation

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Date:   Thu, 5 Nov 2020 16:57:08 +1100
From:   "Anand K. Mistry" <amistry@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-fsdevel@...r.kernel.org,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>, tglx@...utronix.de,
        Alexey Dobriyan <adobriyan@...il.com>,
        Alexey Gladkov <gladkov.alexey@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mauro Carvalho Chehab <mchehab+huawei@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Mike Rapoport <rppt@...nel.org>, NeilBrown <neilb@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc: Provide details on indirect branch speculation

On Sun, 1 Nov 2020 at 07:05, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Fri, 30 Oct 2020 17:27:54 +1100 Anand K Mistry <amistry@...gle.com> wrote:
>
> > Similar to speculation store bypass, show information about the indirect
> > branch speculation mode of a task in /proc/$pid/status.
>
> Why is this considered useful?

For testing/benchmarking, I needed to see whether IB (Indirect Branch)
speculation (see Spectre-v2) is enabled on a task, to see whether an
IBPB instruction should be executed on an address space switch.
Unfortunately, this information isn't available anywhere else and
currently the only way to get it is to hack the kernel to expose it
(like this change). It also helped expose a bug with conditional IB
speculation on certain CPUs.

Another place this could be useful is to audit the system when using
sanboxing. With this change, I can confirm that seccomp-enabled
process have IB speculation force disabled as expected when the kernel
command line parameter `spectre_v2_user=seccomp`.

Since there's already a 'Speculation_Store_Bypass' field, I used that
as precedence for adding this one.

-- 
Anand K. Mistry
Software Engineer
Google Australia