[PATCH v3 3/7] x86/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces

Kees Cook
2020-02-10T19:31:45Z

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@upv.es>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
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 a7035065377c..c9b7be0bcad3 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -287,7 +287,7 @@ extern u32 elf_hwcap2;
*              CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 |
* ELF:              |            |                  |                |
* -------------------------------|------------------|----------------|
- * missing GNU_STACK | exec-all   | exec-all         | exec-all       |
+ * missing GNU_STACK | exec-all   | exec-all         | exec-none      |
* GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     |
* GNU_STACK == RW   | exec-none  | exec-none        | exec-none      |
*
@@ -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