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Date: Tue, 12 Dec 2017 19:41:39 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [patch 11/16] x86/ldt: Force access bit for CS/SS
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@amacapital.net>, Andy Lutomirski <luto@kernel.org>, LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>, Linus Torvalds <torvalds@linux-foundation.org>, Dave Hansen <dave.hansen@intel.com>, Borislav Petkov <bpetkov@suse.de>, Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, Brian Gerst <brgerst@gmail.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Denys Vlasenko <dvlasenk@redhat.com>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Juergen Gross <jgross@suse.com>, David Laight <David.Laight@aculab.com>, Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com, Will Deacon <will.deacon@arm.com>, "linux-mm@kvack.org" <linux-mm@kvack.org>

On Tue, 12 Dec 2017, Peter Zijlstra wrote:

> On Tue, Dec 12, 2017 at 10:22:48AM -0800, Andy Lutomirski wrote:
> > 
> > Also, why is LAR deferred to user exit?  And I thought that LAR didn't
> > set the accessed bit.
> 
> LAR does not set the ACCESSED bit indeed, we need to explicitly set that
> when creating the descriptor.
> 
> It also works if you do the LAR right after LLDT (which is what I
> originally had). The reason its a TIF flag is that I originally LAR'ed
> every entry in the table.
> 
> It got reduced to CS/SS, but the TIF thing stayed.
> 
> > If I had to guess, I'd guess that LAR is actually generating a read
> > fault and forcing the pagetables to get populated.  If so, then it
> > means the VMA code isn't quite right, or you're susceptible to
> > failures under memory pressure.
> > 
> > Now maybe LAR will repopulate the PTE every time if you were to never
> > clear it, but ick.
> 
> I did not observe #PFs from LAR, we had a giant pile of trace_printk()
> in there.

The pages are populated _before_ the new ldt is installed. So no memory
pressure issue, nothing. If the populate fails, then modify_ldt() returns
with an error.

Thanks,

	tglx

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