linux-kernel - Re: Current mainline git (24e700e291d52bd2) hangs when building e.g. perf

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Date:   Mon, 11 Sep 2017 11:08:17 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building
 e.g. perf

On Sun, 2017-09-10 at 18:46 -0700, Andy Lutomirski wrote:
> 
> No, nothing stops the problematic speculative load.  Here's the
> issue.
> One CPU removes a reference to a page table from a higher-level page
> table, flushes, and then frees the page table.  Then it re-allocates
> it and writes something unrelated there.  Another CPU that has CR3
> pointing to the page hierarchy in question could have a reference to
> the freed table in its paging structure cache.  Even if it's
> guaranteed to not try to access the addresses in question (because
> they're user addresses and the other CPU is in kernel mode, etc), but
> there is never a guarantee that the CPU doesn't randomly try to fill
> its TLB for the affected addresses.  This results in invalid PTEs in
> the TLB, possible accesses using bogus memory types, and maybe even
> reads from IO space.

Good point, I had forgotten all about memory accesses
that do not originate with software behavior.