MIPS: Enable VDSO randomization

Based on commit 1091458d09e1a (mmap randomization)

For 32-bit address spaces randomize within a
16MB space, for 64-bit within a 256MB space.

Test Results:
------------
Without Patch (VDSO is not randomized)
---------------------------------------
root@Maleo:~# ./aslr vdso
FAIL: ASLR not functional (vdso always at 0x7fff7000)

root@Maleo:~# ./aslr rekey vdso
pre_val==cur_val
value=0x7fff7000

With patch:(VDSO is randmoized and doesn't interfere with stack)
----------------------------------------------------------------
root@cavium-octeon2:~# ./aslr rekey vdso
pre_val!=cur_val
previous_value=0x7f830ea2
current_value=0x776e2000
root@cavium-octeon2:~# ./aslr rekey vdso
pre_val!=cur_val
previous_value=0x7fb0cea2
current_value=0x77209000
root@cavium-octeon2:~# ./aslr rekey vdso
pre_val!=cur_val
previous_value=0x7f985ea2
current_value=0x7770c000
root@cavium-octeon2:~# ./aslr rekey vdso
pre_val!=cur_val
previous_value=0x7fbc6ea2
current_value=0x7fe25000

Maps file output:
-------------------------
root@cavium-octeon2:~# ./aslr rekey maps
78584000-785a5000 rwxp 00000000 00:00 0                                  [heap]
7f9d0000-7f9f1000 rw-p 00000000 00:00 0                                  [stack]
7ffa5000-7ffa6000 r-xp 00000000 00:00 0                                  [vdso]

root@cavium-octeon2:~# ./aslr rekey maps
77de0000-77e01000 rwxp 00000000 00:00 0                                  [heap]
7f91b000-7f93c000 rw-p 00000000 00:00 0                                  [stack]
7ff99000-7ff9a000 r-xp 00000000 00:00 0                                  [vdso]

root@cavium-octeon2:~# ./aslr rekey maps
77d7f000-77da0000 rwxp 00000000 00:00 0                                  [heap]
7fc2a000-7fc4b000 rw-p 00000000 00:00 0                                  [stack]
7fe09000-7fe0a000 r-xp 00000000 00:00 0                                  [vdso]

root@cavium-octeon2:~# ./aslr rekey maps
7794c000-7794d000 r-xp 00000000 00:00 0                                  [vdso]
77e4b000-77e6c000 rwxp 00000000 00:00 0                                  [heap]
7f6e7000-7f708000 rw-p 00000000 00:00 0                                  [stack]
root@cavium-octeon2:~#

Signed-off-by: Prem Karat <pkarat@mvista.com>
Cc: linux-mips@linux-mips.org
Cc: sergei.shtylyov@cogentembedded.com
Cc: ddaney.cavm@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/6812
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/vdso.c

@@ -16,9 +16,11 @@
 #include <linux/elf.h>
 #include <linux/vmalloc.h>
 #include <linux/unistd.h>
+#include <linux/random.h>
 
 #include <asm/vdso.h>
 #include <asm/uasm.h>
+#include <asm/processor.h>
 
 /*
  * Including <asm/unistd.h> would give use the 64-bit syscall numbers ...
@@ -67,7 +69,18 @@ subsys_initcall(init_vdso);
 
 static unsigned long vdso_addr(unsigned long start)
 {
-	return STACK_TOP;
+	unsigned long offset = 0UL;
+
+	if (current->flags & PF_RANDOMIZE) {
+		offset = get_random_int();
+		offset <<= PAGE_SHIFT;
+		if (TASK_IS_32BIT_ADDR)
+			offset &= 0xfffffful;
+		else
+			offset &= 0xffffffful;
+	}
+
+	return STACK_TOP + offset;
 }
 
 int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)