linux-kernel - Re: [patch V163 27/51] x86/mm/pti: Populate user PGD

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Date:   Wed, 20 Dec 2017 20:50:11 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Juergen Gross <jgross@...e.com>
cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>, keescook@...gle.com,
        hughd@...gle.com, Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>, daniel.gruss@...k.tugraz.at,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch V163 27/51] x86/mm/pti: Populate user PGD

On Wed, 20 Dec 2017, Juergen Gross wrote:
> On 20/12/17 01:22, Thomas Gleixner wrote:
> > On Tue, 19 Dec 2017, Thomas Gleixner wrote:
> >> On Tue, 19 Dec 2017, Ingo Molnar wrote:
> >> We don't run out of space, but the 0-day robot triggered a nasty issue.
> >>
> >> The fixmap bottom address, which contains the early_ioremap fixmap area, is:
> >>
> >>     vaddr_bt = FIXADDR_TOP - FIX_BTMAP_BEGIN * PAGE_SIZE
> >>
> >> If that address is lower than:
> >>
> >>     vaddr_end = __START_KERNEL_map + KERNEL_IMAGE_SIZE;
> >>
> >> then cleanup_highmap() will happily 0 out the PMD entry for the PTE page of
> >> FIX_BTMAP. That entry was set up earlier in early_ioremap_init().
> >>
> >> As a consequence the first call to __early_set_fixmap() which tries to
> >> install a PTE for early_ioremap() will crash and burn.
> >>
> >> Below is a nasty hack which fixes the problem. Ideally we get all of this
> >> cpu_entry_stuff out of the fixmap. I'll look into that later, but for now
> >> the patch 'fixes' the issue.
> > 
> > I had a stab on moving the cpu_entry_area to some other place.
> > 
> > The patch below works, but:
> > 
> >  - it breaks i386 build because I have not yet found a way to place the
> >    CPU_ENTRY_AREA_BASE without creating include recursion hell
> > 
> >  - it probably does not work on XEN_PV, but I'm too tired now to figure
> >    that out.
> 
> The attached patch lets the system come up as XEN_PV.

I folded it back to the proper place.

Thanks for looking!

       tglx