Date: Fri, 5 Jan 2018 15:54:35 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andrea Arcangeli <aarcange@redhat.com>
cc: "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Woodhouse <dwmw2@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/7] IBRS patch series
In-Reply-To: <20180105144340.GF26807@redhat.com>
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On Fri, 5 Jan 2018, Andrea Arcangeli wrote:
> On Thu, Jan 04, 2018 at 09:22:34PM +0000, Van De Ven, Arjan wrote:
> > personally I am comfortable with retpoline on Skylake, but I would
> >like to have IBRS as an opt in for the paranoid.
> 
> I think this whole variant#2 issue has to be fixed mathematically or
> not at all, the reason is that it's already "near zero risk" to begin
> with.
> 
> If we leave any theoretical issues open we can as well stick with the
> "near zero risk" wording on AMD website and not fix anything.
> 
> Doing a huge amount of work with reptoline and then you find SMM is
> called reproducibly somehow and a new PoC could exist for it, not fun.
> 
> Whatever solution should be total and guaranteed or there's huge
> amount of resources wasted.
> 
> Again spectre variant#2 this is "near zero risk" to begin with, on all
> CPUs because of the complexity to mount an attack on any given
> kernel/CPU combination out there. I'm mostly thinking about the "guest
> mode"/"host kernel mod" isolation here but this should apply in
> general to all kernel protection for spectre variant#2.
> 
> If we can do a 3 way alternative IBRS on skylake, repotline + IBRS
> around all BIOS, graphics firmware and pci firmware + kernel asm
> patched with a 2-way alternative between amd reptoline and intel
> reptoline emitted 2-way by same gcc build, and still guarantee this
> solution is as mathematically safe as a pure IBRS (which is obviously
> mathematically safe), ok. Otherwise it'd prefer to stick to pure IBRS
> and skip the reptoline complexity and fallouts here and there around
> bios asm firmware SMM etc.. not to tell the need to rebuild qemu with
> reptoline with 2-way amd/intel alternatives self modifying code in
> userland during qemu startup to achieve ibrs 1 ibpb 1 + prctl IBRS on
> qemu with repotline.

I agree. The amount of options we get plus the desire to do prctl/cgroup
and whatever based enable/disable makes me nervous as hell. Especially the
per task/process/cgroup runtime magic is bound to dig yet another hole of
speculation attack vector into the existing mess.

Thanks,

	tglx