linux-kernel - Re: [patch 56/60] x86/mm/kpti: Disable native VSYSCALL

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Date:   Mon, 4 Dec 2017 14:33:31 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Rik van Riel <riel@...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>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, moritz.lipp@...k.tugraz.at,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Borislav Petkov <bp@...en8.de>, michael.schwarz@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch 56/60] x86/mm/kpti: Disable native VSYSCALL

On Mon, Dec 4, 2017 at 6:08 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> The KERNEL_PAGE_TABLE_ISOLATION code attempts to "poison" the user
> portion of the kernel page tables. It detects entries that it wants that it
> wants to poison in two ways:
>
>  * Looking for addresses >= PAGE_OFFSET
>
>  * Looking for entries without _PAGE_USER set
>
> But, to allow the _PAGE_USER check to work, it must never be set on
> init_mm entries, and an earlier patch in this series ensured that it
> will never be set.
>
> The VDSO is at a address >= PAGE_OFFSET and it is also mapped by init_mm.
> Because of the earlier, KERNEL_PAGE_TABLE_ISOLATION-enforced restriction,
> _PAGE_USER is never set which makes the VDSO unreadable to userspace.
>
> This makes the "NATIVE" case totally unusable since userspace can not even
> see the memory any more.  Disable it whenever KERNEL_PAGE_TABLE_ISOLATION
> is enabled.
>
> Also add some help text about how KERNEL_PAGE_TABLE_ISOLATION might
> affect the emulation case as well.
>

I think my other suggestion may obsolete this patch.