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Subject[PATCH] task_struct: Only use anon struct under randstruct plugin
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.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Fixes: 29e48ce87f1e ("task_struct: Allow randomized")
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
include/linux/compiler-clang.h | 3 ---
include/linux/compiler-gcc.h | 12 +++---------
2 files changed, 3 insertions(+), 12 deletions(-)

diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
index d3f264a5b04d..ceb96ecab96e 100644
--- 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

diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index e2c7f4369eff..b4bf73f5e38f 100644
--- 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 */


--
2.7.4

--
Kees Cook
Pixel Security

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