android: binder: More offset validation

Make sure offsets don't point to overlapping flat_binder_object
structs.

Cc: Colin Cross <ccross@android.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Rom Lemarchand <romlem@google.com>
Cc: Serban Constantinescu <serban.constantinescu@arm.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder.c

@@ -1321,6 +1321,7 @@ static void binder_transaction(struct binder_proc *proc,
 	struct binder_transaction *t;
 	struct binder_work *tcomplete;
 	binder_size_t *offp, *off_end;
+	binder_size_t off_min;
 	struct binder_proc *target_proc;
 	struct binder_thread *target_thread = NULL;
 	struct binder_node *target_node = NULL;
@@ -1522,18 +1523,24 @@ static void binder_transaction(struct binder_proc *proc,
 		goto err_bad_offset;
 	}
 	off_end = (void *)offp + tr->offsets_size;
+	off_min = 0;
 	for (; offp < off_end; offp++) {
 		struct flat_binder_object *fp;
 
 		if (*offp > t->buffer->data_size - sizeof(*fp) ||
+		    *offp < off_min ||
 		    t->buffer->data_size < sizeof(*fp) ||
 		    !IS_ALIGNED(*offp, sizeof(u32))) {
-			binder_user_error("%d:%d got transaction with invalid offset, %lld\n",
-					  proc->pid, thread->pid, (u64)*offp);
+			binder_user_error("%d:%d got transaction with invalid offset, %lld (min %lld, max %lld)\n",
+					  proc->pid, thread->pid, (u64)*offp,
+					  (u64)off_min,
+					  (u64)(t->buffer->data_size -
+					  sizeof(*fp)));
 			return_error = BR_FAILED_REPLY;
 			goto err_bad_offset;
 		}
 		fp = (struct flat_binder_object *)(t->buffer->data + *offp);
+		off_min = *offp + sizeof(struct flat_binder_object);
 		switch (fp->type) {
 		case BINDER_TYPE_BINDER:
 		case BINDER_TYPE_WEAK_BINDER: {