linux-kernel - Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20170811123534.quyckfyspl5fqdrg@hirez.programming.kicks-ass.net>
Date:   Fri, 11 Aug 2017 14:35:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Simon Xiao <sixiao@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>, boris.ostrovsky@...cle.com,
        xen-devel@...ts.xenproject.org
Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB
 flush

On Fri, Aug 11, 2017 at 02:22:25PM +0200, Juergen Gross wrote:
> Wait - the TLB can be cleared at any time, as Andrew was pointing out.
> No cpu can rely on an address being accessible just because IF is being
> cleared. All that matters is the existing and valid page table entry.
> 
> So clearing IF on a cpu isn't meant to secure the TLB from being
> cleared, but just to avoid interrupts (as the name of the flag is
> suggesting).

Yes, but by holding off the TLB invalidate IPI, we hold off the freeing
of the concurrently unhooked page-table.

> In the Xen case the hypervisor does the following:
> 
> - it checks whether any of the vcpus specified in the cpumask of the
>   flush request is running on any physical cpu
> - if any running vcpu is found an IPI will be sent to the physical cpu
>   and the hypervisor will do the TLB flush there

And this will preempt a vcpu which could have IF cleared, right?

> - any vcpu addressed by the flush and not running will be flagged to
>   flush its TLB when being scheduled the next time
> 
> This ensures no TLB entry to be flushed can be used after return of
> xen_flush_tlb_others().

But that is not a sufficient guarantee. We need the IF to hold off the
TLB invalidate and thereby hold off the freeing of our page-table pages.