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

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Date:	Wed, 11 May 2016 12:45:02 +0200
From:	Hector Marco-Gisbert <hecmargi@....es>
To:	linux-kernel@...r.kernel.org
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Brian Gerst <brgerst@...il.com>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Huaitong Han <huaitong.han@...el.com>,
	kees Cook <keescook@...omium.org>,
	Ismael Ripoll Ripoll <iripoll@....es>,
	Hector Marco-Gisbert <hecmargi@....es>
Subject: [PATCH] x86_64: Disabling read-implies-exec when the stack is executable

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