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

Kees Cook
2019-04-18T07:45:39Z

On Wed, May 11, 2016 at 5:45 AM Hector Marco-Gisbert <hecmargi@upv.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@upv.es>
> Acked-by: Ismael Ripoll Ripoll <iripoll@upv.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[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) ? \
+ (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

[1] https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com

--
Kees Cook