linux-kernel - [PATCH v5 3/6] x86/elf: Disable automatic READ_IMPLIES

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Message-Id: <20200327064820.12602-4-keescook@chromium.org>
Date:   Thu, 26 Mar 2020 23:48:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Kees Cook <keescook@...omium.org>,
        Hector Marco-Gisbert <hecmargi@....es>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Catalin Marinas <catalin.marinas@....com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>, Jann Horn <jannh@...gle.com>,
        x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
        kernel-hardening@...ts.openwall.com, linux-kernel@...r.kernel.org
Subject: [PATCH v5 3/6] x86/elf: Disable automatic READ_IMPLIES_EXEC for 64-bit address spaces

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@....es>
Signed-off-by: Kees Cook <keescook@...omium.org>
Reviewed-by: Jason Gunthorpe <jgg@...lanox.com>
---
 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;
  *                 CPU: | lacks NX*  | has NX, ia32     | has NX, x86_64 |
  * ELF:                 |            |                  |                |
  * ---------------------|------------|------------------|----------------|
- * missing PT_GNU_STACK | exec-all   | exec-all         | exec-all       |
+ * missing PT_GNU_STACK | exec-all   | exec-all         | exec-none      |
  * PT_GNU_STACK == RWX  | exec-stack | exec-stack       | exec-stack     |
  * PT_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