linux-kernel - Re: [patch V2 04/25] x86/apic: Make apic_pending_intr

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Date:   Sun, 7 Jul 2019 10:27:29 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     Andrew Cooper <andrew.cooper3@...rix.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Nadav Amit <namit@...are.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Feng Tang <feng.tang@...el.com>
Subject: Re: [patch V2 04/25] x86/apic: Make apic_pending_intr_clear() more
 robust

On Fri, 5 Jul 2019, Andy Lutomirski wrote:
> On Fri, Jul 5, 2019 at 1:36 PM Thomas Gleixner <tglx@...utronix.de> wrote:
> > No. We can map the APIC into the user space visible page tables for PTI
> > without compromising the PTI isolation and it can be read very early on
> > before SWAPGS. All you need is a register to clobber not more. It the ISR
> > is set, then go into an error path, yell loudly, issue EOI and return.
> > The only issue I can see is: It's slow :)
> >
> I think this will be really extremely slow.  If we can restrict this
> to x2apic machines, then maybe it's not so awful.

x2apic machines have working iommu/interrupt remapping.

> FWIW, if we just patch up the GS thing, then we are still vulnerable:
> the bad guy can arrange for a privileged process to have register
> state corresponding to a dangerous syscall and then send an int $0x80
> via the APIC.

Right, that's why you want to read the APIC:ISR to check where that thing
came from.

Thanks,

	tglx