[PATCH v3 4/6] ANDROID: binder: add RT inheritance flag to node.

[Martijn Coenen]

Allows a binder node to specify whether it wants to
inherit real-time scheduling policy from a caller. This
inheritance may not always be desirable, for example in
cases where the binder call runs untrusted and therefore
potentially unbounded code.

Signed-off-by: Martijn Coenen <maco@xxxxxxxxxxx>
---
 drivers/android/binder.c            | 21 +++++++++++++++------
 include/uapi/linux/android/binder.h |  8 ++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 4c42e268b1a5..5958a0876fe8 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -358,6 +358,8 @@ struct binder_error {
  *                        (invariant after initialized)
  * @min_priority:         minimum scheduling priority
  *                        (invariant after initialized)
+ * @inherit_rt:           inherit RT scheduling policy from caller
+ *                        (invariant after initialized)
  * @async_todo:           list of async work items
  *                        (protected by @proc->inner_lock)
  *
@@ -394,6 +396,7 @@ struct binder_node {
 		 * invariant after initialization
 		 */
 		u8 sched_policy:2;
+		u8 inherit_rt:1;
 		u8 accept_fds:1;
 		u8 min_priority;
 	};
@@ -1214,9 +1217,10 @@ static void binder_set_priority(struct task_struct *task,
 
 static void binder_transaction_priority(struct task_struct *task,
 					struct binder_transaction *t,
-					struct binder_priority node_prio)
+					struct binder_priority node_prio,
+					bool inherit_rt)
 {
-	struct binder_priority desired_prio;
+	struct binder_priority desired_prio = t->priority;
 
 	if (t->set_priority_called)
 		return;
@@ -1225,8 +1229,10 @@ static void binder_transaction_priority(struct task_struct *task,
 	t->saved_priority.sched_policy = task->policy;
 	t->saved_priority.prio = task->normal_prio;
 
-	desired_prio.prio = t->priority.prio;
-	desired_prio.sched_policy = t->priority.sched_policy;
+	if (!inherit_rt && is_rt_policy(desired_prio.sched_policy)) {
+		desired_prio.prio = NICE_TO_PRIO(0);
+		desired_prio.sched_policy = SCHED_NORMAL;
+	}
 
 	if (node_prio.prio < t->priority.prio ||
 	    (node_prio.prio == t->priority.prio &&
@@ -1332,6 +1338,7 @@ static struct binder_node *binder_init_node_ilocked(
 		FLAT_BINDER_FLAG_SCHED_POLICY_SHIFT;
 	node->min_priority = to_kernel_prio(node->sched_policy, priority);
 	node->accept_fds = !!(flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
+	node->inherit_rt = !!(flags & FLAT_BINDER_FLAG_INHERIT_RT);
 	spin_lock_init(&node->lock);
 	INIT_LIST_HEAD(&node->work.entry);
 	INIT_LIST_HEAD(&node->async_todo);
@@ -2749,7 +2756,8 @@ static bool binder_proc_transaction(struct binder_transaction *t,
 
 	if (thread) {
 		target_list = &thread->todo;
-		binder_transaction_priority(thread->task, t, node_prio);
+		binder_transaction_priority(thread->task, t, node_prio,
+					    node->inherit_rt);
 	} else if (!target_list) {
 		target_list = &proc->todo;
 	} else {
@@ -4173,7 +4181,8 @@ static int binder_thread_read(struct binder_proc *proc,
 			tr.cookie =  target_node->cookie;
 			node_prio.sched_policy = target_node->sched_policy;
 			node_prio.prio = target_node->min_priority;
-			binder_transaction_priority(current, t, node_prio);
+			binder_transaction_priority(current, t, node_prio,
+						    target_node->inherit_rt);
 			cmd = BR_TRANSACTION;
 		} else {
 			tr.target.ptr = 0;
diff --git a/include/uapi/linux/android/binder.h b/include/uapi/linux/android/binder.h
index b3bced80adea..5539933b3491 100644
--- a/include/uapi/linux/android/binder.h
+++ b/include/uapi/linux/android/binder.h
@@ -79,6 +79,14 @@ enum flat_binder_object_flags {
 	 */
 	FLAT_BINDER_FLAG_SCHED_POLICY_MASK =
 		3U << FLAT_BINDER_FLAG_SCHED_POLICY_SHIFT,
+
+	/**
+	 * @FLAT_BINDER_FLAG_INHERIT_RT: whether the node inherits RT policy
+	 *
+	 * Only when set, calls into this node will inherit a real-time
+	 * scheduling policy from the caller (for synchronous transactions).
+	 */
+	FLAT_BINDER_FLAG_INHERIT_RT = 0x800,
 };
 
 #ifdef BINDER_IPC_32BIT
-- 
2.15.0.rc2.357.g7e34df9404-goog