Re: [PATCH] x86/speculation: Allow overriding seccomp speculation disable

From: Andy Lutomirski <luto@kernel.org>
To: Kees Cook <keescook@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Andi Kleen <andi@firstfloor.org>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@linux.intel.com>, Will Drewry <wad@chromium.org>
Subject: Re: [PATCH] x86/speculation: Allow overriding seccomp speculation disable
Date: Sat, 21 Mar 2020 21:07:07 -0700	[thread overview]
Message-ID: <CALCETrVYSBBtjyBSsHYirKf5eL+YTcLJAnh3W2krxU+uMET8uA@mail.gmail.com> (raw)
In-Reply-To: <202003211916.8078081E0@keescook>

On Sat, Mar 21, 2020 at 7:29 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Sat, Mar 21, 2020 at 03:46:29PM +0100, Thomas Gleixner wrote:
> > Cc+: Seccomp maintainers ....
>
> Thanks!
>
> > Andi Kleen <andi@firstfloor.org> writes:
> > > [...]
> > >
> > > Longer term we probably need to discuss if the seccomp heuristic
> > > is still warranted and should be perhaps changed. It seemed
> > > like a good idea when these vulnerabilities were new, and
> > > no web browsers supported site isolation. But with site isolation
> > > widely deployed -- Chrome has it on by default, and as I understand
> > > it, Firefox is going to enable it by default soon. And other seccomp
> > > users (like sshd or systemd) probably don't really need it.
> > > Given that it's not clear the default heuristic is still a good
> > > idea.
> > >
> > > But anyways this patch doesn't change any defaults, just
> > > let's applications override it.
> >
> > It changes the enforcement and I really want the seccomp people to have
> > a say here.
>
> None of this commit makes sense to me. :)
>
> The point of the defaults was to grandfather older seccomp users into
> speculation mitigations. Newly built seccomp users can choose to disable
> this with SECCOMP_FILTER_FLAG_SPEC_ALLOW when applying seccomp filters.
> The rationale was that once a process knows how to manage its exposure,
> it can choose to leave off the automatic enabling. I don't see any
> mention of that method in the commit log, so if there is some reason
> it's not workable, that would need to be discussed first.

Agreed.

>
> And the force disable matches the design goals of seccomp: no applied
> restrictions can be later relaxed for a process. I'm more in favor of
> changing the behavior of SPEC_STORE_BYPASS_CMD_AUTO, but probably not for
> another 3 years at least. (To get us to at least 5 years since Meltdown,
> which is relatively close to various longer LTS cycles.)
>
> -Kees
>
> --
> Kees Cook