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Date:   Thu, 18 Apr 2019 10:17:21 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Kees Cook <keescook@chromium.org>
cc:     Hector Marco-Gisbert <hecmargi@upv.es>,
        LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Huaitong Han <huaitong.han@intel.com>,
        Ismael Ripoll Ripoll <iripoll@upv.es>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Andi Kleen <ak@linux.intel.com>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH] x86_64: Disabling read-implies-exec when the stack is
 executable
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On Thu, 18 Apr 2019, Kees Cook wrote:
> On Wed, May 11, 2016 at 5:45 AM Hector Marco-Gisbert <hecmargi@upv.es> wrote:
> *thread necromancy*
> 
> I'd still like to see this get landed. READ_IMPLIES_EXEC is way too
> powerful (it impacts, for example, mmap() regions of device driver
> memory, forcing drivers to not be able to disallow VM_EXEC[1]).
> 
> The only case it could break is on an AMD K8 (Athlon only, I assume?),
> which seems unlikely to have a modern kernel run on it. If there is
> still concern, then we could just test against the NX CPU feature:
> 
> diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> index 69c0f892e310..367cd36259a4 100644
> --- a/arch/x86/include/asm/elf.h
> +++ b/arch/x86/include/asm/elf.h
> @@ -280,10 +280,12 @@ extern u32 elf_hwcap2;
> 
>  /*
>   * An executable for which elf_read_implies_exec() returns TRUE will
> - * have the READ_IMPLIES_EXEC personality flag set automatically.
> + * have the READ_IMPLIES_EXEC personality flag set automatically when
> + * a CPU did not support NX or is using a 32-bit memory layout.
>   */
> -#define elf_read_implies_exec(ex, executable_stack)    \
> -       (executable_stack != EXSTACK_DISABLE_X)
> +#define elf_read_implies_exec(ex, executable_stack)            \
> +       (mmap_is_ia32() || !(__supported_pte_mask & _PAGE_NX) ? \

What's special about ia32? All what matters is whether PAGE_NX is supported
or not. That has nothing to do with 32/64bit unless I'm missing something
(as usual).

Thanks,

	tglx