Re: [PATCH] x86_64: Disabling read-implies-exec when the stack is executable

Thomas Gleixner
2019-04-18T08:17:28Z

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