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From:   Kees Cook <keescook@google.com>
Date:   Mon, 23 Apr 2018 10:09:19 -0700
Message-ID: <CAGXu5jKYhqvgooq8q-2NoMC_Cqh-SR8J0y0c1x9LteinDfQELQ@mail.gmail.com>
Subject: Re: [PATCH 28/37] x86/mm/pti: Map kernel-text to user-space on 32 bit kernels
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
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        Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
        Hugh Dickins <hughd@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        "David H . Gutteridge" <dhgutteridge@sympatico.ca>,
        Joerg Roedel <jroedel@suse.de>
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On Mon, Apr 23, 2018 at 8:47 AM, Joerg Roedel <joro@8bytes.org> wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> Keeping the kernel text mapped with G bit set keeps its
> entries in the TLB across kernel entry/exit and improved the
> performance. The 64 bit x86 kernels already do this when
> there is no PCID, so do this in 32 bit as well since PCID is
> not even supported there.

I think this should keep at least part of the logic as 64-bit since
there are other reasons to turn off the Global flag:

https://lkml.kernel.org/r/20180420222026.D0B4AAC9@viggo.jf.intel.com

-Kees

>
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
>  arch/x86/mm/init_32.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/mm/init_32.c b/arch/x86/mm/init_32.c
> index c893c6a..8299b98 100644
> --- a/arch/x86/mm/init_32.c
> +++ b/arch/x86/mm/init_32.c
> @@ -956,4 +956,10 @@ void mark_rodata_ro(void)
>         mark_nxdata_nx();
>         if (__supported_pte_mask & _PAGE_NX)
>                 debug_checkwx();
> +
> +       /*
> +        * Do this after all of the manipulation of the
> +        * kernel text page tables are complete.
> +        */
> +       pti_clone_kernel_text();
>  }
> --
> 2.7.4
>


-- 
Kees Cook
Pixel Security