[PATCH] binder: fix use-after-free in binder_transaction()

[Todd Kjos]

User-space normally keeps the node alive when creating a transaction
since it has a reference to the target. The local strong ref keeps it
alive if the sending process dies before the target process processes
the transaction. If the source process is malicious or has a reference
counting bug, this can fail.

In this case, when we attempt to decrement the node in the failure
path, the node has already been freed.

This is fixed by taking a tmpref on the node while constructing
the transaction. To avoid re-acquiring the node lock and inner
proc lock to increment the proc's tmpref, a helper is used that
does the ref increments on both the node and proc.

Signed-off-by: Todd Kjos <tkjos@xxxxxxxxxx>
---
 drivers/android/binder.c | 93 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 66 insertions(+), 27 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index d055b3f2a207..40e8a58391cc 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -2583,6 +2583,48 @@ static bool binder_proc_transaction(struct binder_transaction *t,
 	return true;
 }
 
+/**
+ * binder_get_node_refs_for_txn() - Get required refs on node for txn
+ * @node:         struct binder_node for which to get refs
+ * @proc:         returns @node->proc if valid
+ * @error:        if no @proc then returns BR_DEAD_REPLY
+ *
+ * User-space normally keeps the node alive when creating a transaction
+ * since it has a reference to the target. The local strong ref keeps it
+ * alive if the sending process dies before the target process processes
+ * the transaction. If the source process is malicious or has a reference
+ * counting bug, relying on the local strong ref can fail.
+ *
+ * Since user-space can cause the local strong ref to go away, we also take
+ * a tmpref on the node to ensure it survives while we are constructing
+ * the transaction. We also need a tmpref on the proc while we are
+ * constructing the transaction, so we take that here as well.
+ *
+ * Return: The target_node with refs taken or NULL if no @node->proc is NULL.
+ * Also sets @proc if valid. If the @node->proc is NULL indicating that the
+ * target proc has died, @error is set to BR_DEAD_REPLY
+ */
+static struct binder_node *binder_get_node_refs_for_txn(
+		struct binder_node *node,
+		struct binder_proc **procp,
+		uint32_t *error)
+{
+	struct binder_node *target_node = NULL;
+
+	binder_node_inner_lock(node);
+	if (node->proc) {
+		target_node = node;
+		binder_inc_node_nilocked(node, 1, 0, NULL);
+		binder_inc_node_tmpref_ilocked(node);
+		node->proc->tmp_ref++;
+		*procp = node->proc;
+	} else
+		*error = BR_DEAD_REPLY;
+	binder_node_inner_unlock(node);
+
+	return target_node;
+}
+
 static void binder_transaction(struct binder_proc *proc,
 			       struct binder_thread *thread,
 			       struct binder_transaction_data *tr, int reply,
@@ -2686,43 +2728,35 @@ static void binder_transaction(struct binder_proc *proc,
 			ref = binder_get_ref_olocked(proc, tr->target.handle,
 						     true);
 			if (ref) {
-				binder_inc_node(ref->node, 1, 0, NULL);
-				target_node = ref->node;
-			}
-			binder_proc_unlock(proc);
-			if (target_node == NULL) {
+				target_node = binder_get_node_refs_for_txn(
+						ref->node, &target_proc,
+						&return_error);
+			} else {
 				binder_user_error("%d:%d got transaction to invalid handle\n",
-					proc->pid, thread->pid);
+						  proc->pid, thread->pid);
 				return_error = BR_FAILED_REPLY;
-				return_error_param = -EINVAL;
-				return_error_line = __LINE__;
-				goto err_invalid_target_handle;
 			}
+			binder_proc_unlock(proc);
 		} else {
 			mutex_lock(&context->context_mgr_node_lock);
 			target_node = context->binder_context_mgr_node;
-			if (target_node == NULL) {
+			if (target_node)
+				target_node = binder_get_node_refs_for_txn(
+						target_node, &target_proc,
+						&return_error);
+			else
 				return_error = BR_DEAD_REPLY;
-				mutex_unlock(&context->context_mgr_node_lock);
-				return_error_line = __LINE__;
-				goto err_no_context_mgr_node;
-			}
-			binder_inc_node(target_node, 1, 0, NULL);
 			mutex_unlock(&context->context_mgr_node_lock);
 		}
-		e->to_node = target_node->debug_id;
-		binder_node_lock(target_node);
-		target_proc = target_node->proc;
-		if (target_proc == NULL) {
-			binder_node_unlock(target_node);
-			return_error = BR_DEAD_REPLY;
+		if (!target_node) {
+			/*
+			 * return_error is set above
+			 */
+			return_error_param = -EINVAL;
 			return_error_line = __LINE__;
 			goto err_dead_binder;
 		}
-		binder_inner_proc_lock(target_proc);
-		target_proc->tmp_ref++;
-		binder_inner_proc_unlock(target_proc);
-		binder_node_unlock(target_node);
+		e->to_node = target_node->debug_id;
 		if (security_binder_transaction(proc->tsk,
 						target_proc->tsk) < 0) {
 			return_error = BR_FAILED_REPLY;
@@ -3072,6 +3106,8 @@ static void binder_transaction(struct binder_proc *proc,
 	if (target_thread)
 		binder_thread_dec_tmpref(target_thread);
 	binder_proc_dec_tmpref(target_proc);
+	if (target_node)
+		binder_dec_node_tmpref(target_node);
 	/*
 	 * write barrier to synchronize with initialization
 	 * of log entry
@@ -3090,6 +3126,8 @@ static void binder_transaction(struct binder_proc *proc,
 err_copy_data_failed:
 	trace_binder_transaction_failed_buffer_release(t->buffer);
 	binder_transaction_buffer_release(target_proc, t->buffer, offp);
+	if (target_node)
+		binder_dec_node_tmpref(target_node);
 	target_node = NULL;
 	t->buffer->transaction = NULL;
 	binder_alloc_free_buf(&target_proc->alloc, t->buffer);
@@ -3104,13 +3142,14 @@ static void binder_transaction(struct binder_proc *proc,
 err_empty_call_stack:
 err_dead_binder:
 err_invalid_target_handle:
-err_no_context_mgr_node:
 	if (target_thread)
 		binder_thread_dec_tmpref(target_thread);
 	if (target_proc)
 		binder_proc_dec_tmpref(target_proc);
-	if (target_node)
+	if (target_node) {
 		binder_dec_node(target_node, 1, 0);
+		binder_dec_node_tmpref(target_node);
+	}
 
 	binder_debug(BINDER_DEBUG_FAILED_TRANSACTION,
 		     "%d:%d transaction failed %d/%d, size %lld-%lld line %d\n",
-- 
2.14.2.822.g60be5d43e6-goog