MIPS: Enable heap randomization.

Based somewhat on the PPC implementation.

32-bit processes have the heap randomized in an 8MB space, 256MB for
64-bit processes.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1479/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/elf.h

@@ -372,4 +372,9 @@ extern const char *__elf_platform;
 struct linux_binprm;
 extern int arch_setup_additional_pages(struct linux_binprm *bprm,
 				       int uses_interp);
+
+struct mm_struct;
+extern unsigned long arch_randomize_brk(struct mm_struct *mm);
+#define arch_randomize_brk arch_randomize_brk
+
 #endif /* _ASM_ELF_H */

arch/mips/kernel/syscall.c

@@ -30,6 +30,7 @@
 #include <linux/uaccess.h>
 #include <linux/slab.h>
 #include <linux/random.h>
+#include <linux/elf.h>
 
 #include <asm/asm.h>
 #include <asm/branch.h>
@@ -153,6 +154,33 @@ void arch_pick_mmap_layout(struct mm_struct *mm)
 	mm->unmap_area = arch_unmap_area;
 }
 
+static inline unsigned long brk_rnd(void)
+{
+	unsigned long rnd = get_random_int();
+
+	rnd = rnd << PAGE_SHIFT;
+	/* 8MB for 32bit, 256MB for 64bit */
+	if (TASK_IS_32BIT_ADDR)
+		rnd = rnd & 0x7ffffful;
+	else
+		rnd = rnd & 0xffffffful;
+
+	return rnd;
+}
+
+unsigned long arch_randomize_brk(struct mm_struct *mm)
+{
+	unsigned long base = mm->brk;
+	unsigned long ret;
+
+	ret = PAGE_ALIGN(base + brk_rnd());
+
+	if (ret < mm->brk)
+		return mm->brk;
+
+	return ret;
+}
+
 SYSCALL_DEFINE6(mips_mmap, unsigned long, addr, unsigned long, len,
 	unsigned long, prot, unsigned long, flags, unsigned long,
 	fd, off_t, offset)