[PATCH v5 3/6] x86/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces

[Kees Cook]

With modern x86 64-bit environments, there should never be a need for
automatic READ_IMPLIES_EXEC, as the architecture is intended to always
be execute-bit aware (as in, the default memory protection should be NX
unless a region explicitly requests to be executable).

There were very old x86_64 systems that lacked the NX bit, but for those,
the NX bit is, obviously, unenforceable, so these changes should have
no impact on them.

Suggested-by: Hector Marco-Gisbert <hecmargi@xxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
---
 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 397a1c74433e..452beed7892b 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -287,7 +287,7 @@ extern u32 elf_hwcap2;
  * Â Â Â Â Â Â    ÂCPU: | lacks NX* Â| has NX, ia32 Â Â | has NX, x86_64 |
  * ELF: Â Â Â Â    Â Â Â| Â Â Â Â Â Â| Â Â Â Â Â Â Â Â Â| Â Â Â Â Â Â Â Â|
  * ---------------------|------------|------------------|----------------|
- * missing PT_GNU_STACK | exec-all  | exec-all     | exec-all    |
+ * missing PT_GNU_STACK | exec-all  | exec-all     | exec-none   |
  * PT_GNU_STACK == RWX Â| exec-stack | exec-stack    | exec-stack   |
  * PT_GNU_STACK == RW  | exec-none Â| exec-none    Â| exec-none   Â|
  *
@@ -303,7 +303,7 @@ extern u32 elf_hwcap2;
  *
  */
 #define elf_read_implies_exec(ex, executable_stack)	\
-	(executable_stack == EXSTACK_DEFAULT)
+	(mmap_is_ia32() && executable_stack == EXSTACK_DEFAULT)
 
 struct task_struct;
 
-- 
2.20.1