Date: Thu, 18 Apr 2019 02:45:22 -0500 From: Kees Cook <keescook@...omium.org> To: Hector Marco-Gisbert <hecmargi@....es> Cc: LKML <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, 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 Wed, May 11, 2016 at 5:45 AM Hector Marco-Gisbert <hecmargi@....es> wrote: > > The READ_IMPLIES_EXEC personality was removed in 2005 for 64-bit processes, > (commit a3cc2546a54361b86b73557df5b85c4fc3fc27c3 form history.git). > > But it's still possible to have all readable areas with EXEC permissions by > setting the stack as executable in 64-bit ELF executables (also in 32-bit). > > This is because the macro elf_read_implies_exec() does not distinguish > between 32 and 64-bit executables: when the stack is executable then the > read-implies-exec personality is set (enabled) to the process. > > We think that this is not a desirable behaviour, maybe read-implies-exec > could be used via personality but not by setting the stack as executable in > the ELF. > > For x86_64 processes, is there any reason to disable read-implies-exec > personality and at the same time enable it when the stack is executable ? > > With this patch it's no longer possible to enable the read-implies-exec on > the process by setting the stack as executable in the PT_GNU_STACK on > x86_64 executables. > > Regarding 32-bits processes, is there any reason to enable > read-implies-exec by setting the stack as executable instead of using the > personality on X86_32 or when emulating IA32 on X86_64 ? > > If not, I could re-send the patch which removes this possibility. > > > Signed-off-by: Hector Marco-Gisbert <hecmargi@....es> > Acked-by: Ismael Ripoll Ripoll <iripoll@....es> > --- > arch/x86/include/asm/elf.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h > index 15340e3..87fd15e 100644 > --- a/arch/x86/include/asm/elf.h > +++ b/arch/x86/include/asm/elf.h > @@ -271,8 +271,8 @@ extern int force_personality32; > * An executable for which elf_read_implies_exec() returns TRUE will > * have the READ_IMPLIES_EXEC personality flag set automatically. > */ > -#define elf_read_implies_exec(ex, executable_stack) \ > - (executable_stack != EXSTACK_DISABLE_X) > +#define elf_read_implies_exec(ex, executable_stack) \ > + (mmap_is_ia32() ? (executable_stack != EXSTACK_DISABLE_X) : 0) > > struct task_struct; > > -- > 1.9.1 > *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) ? \ + (executable_stack != EXSTACK_DISABLE_X) : 0) struct task_struct; Additionally, I think architectures that always had NX (arm64) should entirely remove their elf_read_implies_exec() macro (defaulting to the removal of the stack-marking-based READ_IMPLIES_EXEC enabling): diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h index 6adc1a90e7e6..87c2dd468eea 100644 --- a/arch/arm64/include/asm/elf.h +++ b/arch/arm64/include/asm/elf.h @@ -107,8 +107,6 @@ */ #define elf_check_arch(x) ((x)->e_machine == EM_AARCH64) -#define elf_read_implies_exec(ex,stk) (stk != EXSTACK_DISABLE_X) - #define CORE_DUMP_USE_REGSET #define ELF_EXEC_PAGESIZE PAGE_SIZE Thoughts? -Kees  https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com -- Kees Cook
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.