linux-kernel - Re: [patch 28/60] x86/mm/kpti: Disable global pages if KERNEL_PAGE_TABLE

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Date:   Tue, 5 Dec 2017 15:34:53 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, 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>,
        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>,
        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@...k.tugraz.at,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, moritz.lipp@...k.tugraz.at,
        linux-mm@...ck.org, richard.fellner@...dent.tugraz.at,
        michael.schwarz@...k.tugraz.at
Subject: Re: [patch 28/60] x86/mm/kpti: Disable global pages if
 KERNEL_PAGE_TABLE_ISOLATION=y

On Mon, Dec 04, 2017 at 03:07:34PM +0100, Thomas Gleixner wrote:
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> Global pages stay in the TLB across context switches.  Since all contexts
> share the same kernel mapping, these mappings are marked as global pages
> so kernel entries in the TLB are not flushed out on a context switch.
> 
> But, even having these entries in the TLB opens up something that an
> attacker can use, such as the double-page-fault attack:
> 
>    http://www.ieee-security.org/TC/SP2013/papers/4977a191.pdf
> 
> That means that even when KERNEL_PAGE_TABLE_ISOLATION switches page tables
> on return to user space the global pages would stay in the TLB cache.
> 
> Disable global pages so that kernel TLB entries can be flushed before
> returning to user space. This way, all accesses to kernel addresses from
> userspace result in a TLB miss independent of the existence of a kernel
> mapping.
> 
> Supress global pages via the __supported_pte_mask. The user space

"Suppress"

Otherwise

Reviewed-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--