linux-kernel - [PATCH 4.16 012/196] task_struct: only use anon struct under randstruct plugin

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Date:   Sun, 22 Apr 2018 15:50:32 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 4.16 012/196] task_struct: only use anon struct under randstruct plugin

4.16-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@...omium.org>

commit 2cfe0d3009418a132b93d78642a8059a38fe5944 upstream.

The original intent for always adding the anonymous struct in
task_struct was to make sure we had compiler coverage.

However, this caused pathological padding of 40 bytes at the start of
task_struct.  Instead, move the anonymous struct to being only used when
struct layout randomization is enabled.

Link: http://lkml.kernel.org/r/20180327213609.GA2964@beast
Fixes: 29e48ce87f1e ("task_struct: Allow randomized")
Signed-off-by: Kees Cook <keescook@...omium.org>
Reported-by: Peter Zijlstra <peterz@...radead.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 include/linux/compiler-clang.h |    3 ---
 include/linux/compiler-gcc.h   |   12 +++---------
 2 files changed, 3 insertions(+), 12 deletions(-)

--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -17,9 +17,6 @@
  */
 #define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
 
-#define randomized_struct_fields_start	struct {
-#define randomized_struct_fields_end	};
-
 /* all clang versions usable with the kernel support KASAN ABI version 5 */
 #define KASAN_ABI_VERSION 5
 
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -242,6 +242,9 @@
 #if defined(RANDSTRUCT_PLUGIN) && !defined(__CHECKER__)
 #define __randomize_layout __attribute__((randomize_layout))
 #define __no_randomize_layout __attribute__((no_randomize_layout))
+/* This anon struct can add padding, so only enable it under randstruct. */
+#define randomized_struct_fields_start	struct {
+#define randomized_struct_fields_end	} __randomize_layout;
 #endif
 
 #endif /* GCC_VERSION >= 40500 */
@@ -256,15 +259,6 @@
  */
 #define __visible	__attribute__((externally_visible))
 
-/*
- * RANDSTRUCT_PLUGIN wants to use an anonymous struct, but it is only
- * possible since GCC 4.6. To provide as much build testing coverage
- * as possible, this is used for all GCC 4.6+ builds, and not just on
- * RANDSTRUCT_PLUGIN builds.
- */
-#define randomized_struct_fields_start	struct {
-#define randomized_struct_fields_end	} __randomize_layout;
-
 #endif /* GCC_VERSION >= 40600 */