[RFC] User stack pointer randomisation

[Franck Bui-Huu <vagabon.xyz@gmail.com>]

This patch adds a page size range randomisation to the user
stack pointer.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
---

  Hi Ralf,

 This is taken from the x86 architecture. I modified it a bit so the
 randomisation range is only a page size range. Since the top of the
 stack is already randomised, I don't see any point to make the range
 bigger as this is the case in x86 arch. I'm surely missing something
 obvious and that's the reason this patch is a RFC.

 I tested it and it works fine so far.

 Please try to have a look,

		Franck

 arch/mips/kernel/process.c |   13 +++++++++++++
 include/asm-mips/system.h  |    2 +-
 2 files changed, 14 insertions(+), 1 deletions(-)

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index 6bdfb5a..4f411fa 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -25,6 +25,7 @@
 #include <linux/init.h>
 #include <linux/completion.h>
 #include <linux/kallsyms.h>
+#include <linux/random.h>
 
 #include <asm/bootinfo.h>
 #include <asm/cpu.h>
@@ -460,3 +461,15 @@ unsigned long get_wchan(struct task_struct *task)
 out:
 	return pc;
 }
+
+/*
+ * Don't forget that the stack pointer must be aligned on a 8 bytes
+ * boundary at least.
+ */
+unsigned long arch_align_stack(unsigned long sp)
+{
+	if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
+		sp -= get_random_int() & ~PAGE_MASK;
+
+	return sp & ~7;
+}
diff --git a/include/asm-mips/system.h b/include/asm-mips/system.h
index 2908870..0cfb6e1 100644
--- a/include/asm-mips/system.h
+++ b/include/asm-mips/system.h
@@ -355,6 +355,6 @@ extern int stop_a_enabled;
  */
 #define __ARCH_WANT_UNLOCKED_CTXSW
 
-#define arch_align_stack(x) (x)
+extern unsigned long arch_align_stack(unsigned long sp);
 
 #endif /* _ASM_SYSTEM_H */
-- 
1.5.2.2