randomize lower bits of the argument block

This was extracted from the PaX RANDUSTACK feature in grsecurity, where
all of the lower bits are randomized. PaX keeps 16-byte alignment.

Signed-off-by: Daniel Micay <danielmicay@gmail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/exec.c b/fs/exec.c
index ee10ebb57..632f839d5 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -55,6 +55,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/oom.h>
 #include <linux/compat.h>
+#include <linux/random.h>
 
 #include <asm/uaccess.h>
 #include <asm/mmu_context.h>
@@ -297,6 +298,8 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	mm->stack_vm = mm->total_vm = 1;
 	up_write(&mm->mmap_sem);
 	bprm->p = vma->vm_end - sizeof(void *);
+	if (randomize_va_space)
+		bprm->p ^= get_random_long() & ~PAGE_MASK;
 	return 0;
 err:
 	up_write(&mm->mmap_sem);