linux-kernel - Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page tables (core patch)

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Date:   Fri, 5 Jan 2018 20:17:17 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
cc:     Yisheng Xie <xieyisheng1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, richard.fellner@...dent.tugraz.at,
        moritz.lipp@...k.tugraz.at, daniel.gruss@...k.tugraz.at,
        michael.schwarz@...k.tugraz.at, luto@...nel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        keescook@...gle.com, hughd@...gle.com, x86@...nel.org,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 05/23] x86, kaiser: unmap kernel from userspace page
 tables (core patch)

On Fri, 5 Jan 2018, Dave Hansen wrote:

> > --- a/arch/x86/platform/efi/efi_64.c
> > +++ b/arch/x86/platform/efi/efi_64.c
> > @@ -95,6 +95,12 @@ pgd_t * __init efi_call_phys_prolog(void
> >  		save_pgd[pgd] = *pgd_offset_k(pgd * PGDIR_SIZE);
> >  		vaddress = (unsigned long)__va(pgd * PGDIR_SIZE);
> >  		set_pgd(pgd_offset_k(pgd * PGDIR_SIZE), *pgd_offset_k(vaddress));
> > +		/*
> > +		 * pgprot API doesn't clear it for PGD
> > +		 *
> > +		 * Will be brought back automatically in _epilog()
> > +		 */
> > +		pgd_offset_k(pgd * PGDIR_SIZE)->pgd &= ~_PAGE_NX;
> >  	}
> >  	__flush_tlb_all();
> 
> Wait a sec...  Where does the _PAGE_USER come from?  Shouldn't we see
> the &init_mm in there and *not* set _PAGE_USER?

That's because pgd_populate() uses _PAGE_TABLE and not _KERNPG_TABLE for 
reasons that are behind me.

I did put this on my TODO list, but for later.

(and yes, I tried clearing _PAGE_USER from init_mm's PGD, and no obvious 
breakages appeared, but I wanted to give it more thought later).

-- 
Jiri Kosina
SUSE Labs