Date: Thu, 18 Apr 2019 10:17:21 +0200 (CEST) From: Thomas Gleixner <tglx@...utronix.de> To: Kees Cook <keescook@...omium.org> cc: Hector Marco-Gisbert <hecmargi@....es>, LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>, Brian Gerst <brgerst@...il.com>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Huaitong Han <huaitong.han@...el.com>, Ismael Ripoll Ripoll <iripoll@....es>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Jason Gunthorpe <jgg@...lanox.com>, Andi Kleen <ak@...ux.intel.com>, Mark Rutland <mark.rutland@....com> Subject: Re: [PATCH] x86_64: Disabling read-implies-exec when the stack is executable On Thu, 18 Apr 2019, Kees Cook wrote: > On Wed, May 11, 2016 at 5:45 AM Hector Marco-Gisbert <hecmargi@....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). > > 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
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