binder: don't allow mmap() by process other than proc->tsk

we really shouldn't do get_files_struct() on a different process
and use it to modify the sucker later on.
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

drivers/staging/android/binder.c

@@ -2793,6 +2793,9 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 	const char *failure_string;
 	struct binder_buffer *buffer;
 
+	if (proc->tsk != current)
+		return -EINVAL;
+
 	if ((vma->vm_end - vma->vm_start) > SZ_4M)
 		vma->vm_end = vma->vm_start + SZ_4M;
 
@@ -2857,7 +2860,7 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 	binder_insert_free_buffer(proc, buffer);
 	proc->free_async_space = proc->buffer_size / 2;
 	barrier();
-	proc->files = get_files_struct(proc->tsk);
+	proc->files = get_files_struct(current);
 	proc->vma = vma;
 	proc->vma_vm_mm = vma->vm_mm;