[PATCH v4 04/10] mips: extract logic for mmap_rnd()

Kees Cook
2015-03-04T21:13:20Z

In preparation for splitting out ET_DYN ASLR, extract the mmap ASLR
selection into a separate function.

Signed-off-by: Kees Cook <keescook@chromium.org>
---
It seems the entropy gets smaller as the PAGE_SIZE increases. Is this
intentional?
---
arch/mips/mm/mmap.c | 24 ++++++++++++++++--------
1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/arch/mips/mm/mmap.c b/arch/mips/mm/mmap.c
index f1baadd56e82..9a4f1f5c1f0e 100644
--- a/arch/mips/mm/mmap.c
+++ b/arch/mips/mm/mmap.c
@@ -142,18 +142,26 @@ unsigned long arch_get_unmapped_area_topdown(struct file *filp,
addr0, len, pgoff, flags, DOWN);
}

+static unsigned long mmap_rnd(void)
+{
+ unsigned long rnd;
+
+ rnd = (unsigned long)get_random_int();
+ rnd <<= PAGE_SHIFT;
+ if (TASK_IS_32BIT_ADDR)
+ rnd &= 0xfffffful;
+ else
+ rnd &= 0xffffffful;
+
+ return rnd;
+}
+
void arch_pick_mmap_layout(struct mm_struct *mm)
{
unsigned long random_factor = 0UL;

- if (current->flags & PF_RANDOMIZE) {
- random_factor = get_random_int();
- random_factor = random_factor << PAGE_SHIFT;
- if (TASK_IS_32BIT_ADDR)
- random_factor &= 0xfffffful;
- else
- random_factor &= 0xffffffful;
- }
+ if (current->flags & PF_RANDOMIZE)
+ random_factor = mmap_rnd();

if (mmap_is_legacy()) {
mm->mmap_base = TASK_UNMAPPED_BASE + random_factor;
--
1.9.1