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Subject: Re: Current mainline git (24e700e291d52bd2) hangs when building
 e.g. perf
From: Rik van Riel <riel@redhat.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Tom Lendacky <thomas.lendacky@amd.com>
Date: Mon, 11 Sep 2017 11:08:17 -0400
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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.