linux-kernel - Re: [patch 11/16] x86/ldt: Force access bit for CS/SS

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Date:   Tue, 12 Dec 2017 19:41:39 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [patch 11/16] x86/ldt: Force access bit for CS/SS

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