android: binder: fix dangling pointer comparison

If /dev/binder is opened and the opener process then e.g. calls execve,
proc->vma_vm_mm will still point to the location of the now-freed
mm_struct. If the process then calls ioctl(binder_fd, ...), the dangling
proc->vma_vm_mm pointer will be compared to current->mm.

Let the binder take a reference to the mm_struct to avoid this.

v2: use the right refcounter

Fixes: a906d693 ("android: binder: Sanity check at binder ioctl")
Signed-off-by: NJann Horn <jannh@google.com>
Reviewed-by: NChen Feng <puck.chen@hisilicon.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -2962,6 +2962,7 @@ static int binder_open(struct inode *nodp, struct file *filp)
 		return -ENOMEM;
 	get_task_struct(current);
 	proc->tsk = current;
+	atomic_inc(&current->mm->mm_count);
 	proc->vma_vm_mm = current->mm;
 	INIT_LIST_HEAD(&proc->todo);
 	init_waitqueue_head(&proc->wait);
@@ -3167,6 +3168,7 @@ static void binder_deferred_release(struct binder_proc *proc)
 		vfree(proc->buffer);
 	}
 
+	mmdrop(proc->vma_vm_mm);
 	put_task_struct(proc->tsk);
 
 	binder_debug(BINDER_DEBUG_OPEN_CLOSE,