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Date: Thu, 24 May 2018 13:19:45 +0100
From: Will Deacon <will.deacon@arm.com>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kvmarm@lists.cs.columbia.edu, Kees Cook <keescook@chromium.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 09/14] arm64: ssbd: Introduce thread flag to control
 userspace mitigation
Message-ID: <20180524121944.GC8689@arm.com>
References: <20180522150648.28297-1-marc.zyngier@arm.com>
 <20180522150648.28297-10-marc.zyngier@arm.com>
 <20180524120121.pjdw7qaybcsbf4fl@lakrids.cambridge.arm.com>
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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@arm.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