linux-kernel - Re: [PATCH 09/14] arm64: ssbd: Introduce thread flag to control userspace mitigation

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Date:   Thu, 24 May 2018 13:19:45 +0100
From:   Will Deacon <will.deacon@....com>
To:     Marc Zyngier <marc.zyngier@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvmarm@...ts.cs.columbia.edu, Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Andy Lutomirski <luto@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 09/14] arm64: ssbd: Introduce thread flag to control
 userspace mitigation

On Thu, May 24, 2018 at 01:16:38PM +0100, Marc Zyngier wrote:
> On 24/05/18 13:01, Mark Rutland wrote:
> > On Tue, May 22, 2018 at 04:06:43PM +0100, Marc Zyngier wrote:
> >> In order to allow userspace to be mitigated on demand, let's
> >> introduce a new thread flag that prevents the mitigation from
> >> being turned off when exiting to userspace, and doesn't turn
> >> it on on entry into the kernel (with the assumtion that the
> > 
> > Nit: s/assumtion/assumption/
> > 
> >> mitigation is always enabled in the kernel itself).
> >>
> >> This will be used by a prctl interface introduced in a later
> >> patch.
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@....com>
> > 
> > On the assumption that this flag cannot be flipped while a task is in
> > userspace:
> 
> Well, that's the case unless you get into the seccomp thing, which does
> change TIF_SSBD on all threads of the task, without taking it to the
> kernel first. That nicely breaks the state machine, and you end-up
> running non-mitigated in the kernel. Oops.
> 
> I have a couple of patches fixing that, using a second flag
> (TIF_SSBD_PENDING) that gets turned into the real thing on exit to
> userspace. It's pretty ugly though.

... which introduces the need for atomics on the entry path too :(

I would /much/ rather kill the seccomp implicit enabling of the mitigation,
or at least have a way to opt-out per arch since it doesn't seem to be
technically justified imo.

Will