TT#14008 fix call iterator race condition

The contents of the ->next element cannot be accessed completely lock
free as they're zeroed out during call removal. Instead grab a reference
to the linked next call before releasing the lock, and also lock the
next element before moving on. This requires a more granular locking as
not to interfere with call removal: One lock to protect the contained
call and the ->next, and another to protect the ->prev

Change-Id: I5474ea3f88e3276f93ba62a952b3be13c0c182e9

daemon/call.c

@@ -2822,21 +2822,22 @@ void call_destroy(struct call *c) {
 		while (1) {
 			mutex_lock(&rtpe_call_iterators[i].lock);
 			// lock this entry
-			mutex_lock(&c->iterator[i].lock);
+			mutex_lock(&c->iterator[i].next_lock);
+			mutex_lock(&c->iterator[i].prev_lock);
 			// try lock adjacent entries
 			prev_call = c->iterator[i].link.prev ? c->iterator[i].link.prev->data : NULL;
 			next_call = c->iterator[i].link.next ? c->iterator[i].link.next->data : NULL;
 			if (prev_call) {
-				if (mutex_trylock(&prev_call->iterator[i].lock)) {
-					mutex_unlock(&c->iterator[i].lock);
+				if (mutex_trylock(&prev_call->iterator[i].next_lock)) {
+					mutex_unlock(&c->iterator[i].next_lock);
 					mutex_unlock(&rtpe_call_iterators[i].lock);
 					continue; // try again
 				}
 			}
 			if (next_call) {
-				if (mutex_trylock(&next_call->iterator[i].lock)) {
-					mutex_unlock(&prev_call->iterator[i].lock);
-					mutex_unlock(&c->iterator[i].lock);
+				if (mutex_trylock(&next_call->iterator[i].prev_lock)) {
+					mutex_unlock(&prev_call->iterator[i].next_lock);
+					mutex_unlock(&c->iterator[i].next_lock);
 					mutex_unlock(&rtpe_call_iterators[i].lock);
 					continue; // try again
 				}
@@ -2849,10 +2850,11 @@ void call_destroy(struct call *c) {
 				&c->iterator[i].link);
 		ZERO(c->iterator[i].link);
 		if (prev_call)
-			mutex_unlock(&prev_call->iterator[i].lock);
+			mutex_unlock(&prev_call->iterator[i].next_lock);
 		if (next_call)
-			mutex_unlock(&next_call->iterator[i].lock);
-		mutex_unlock(&c->iterator[i].lock);
+			mutex_unlock(&next_call->iterator[i].prev_lock);
+		mutex_unlock(&c->iterator[i].next_lock);
+		mutex_unlock(&c->iterator[i].prev_lock);
 		mutex_unlock(&rtpe_call_iterators[i].lock);
 	}
 
@@ -3106,8 +3108,10 @@ static struct call *call_create(const str *callid) {
 	c->dtls_cert = dtls_cert();
 	c->tos = rtpe_config.default_tos;
 
-	for (int i = 0; i < NUM_CALL_ITERATORS; i++)
-		mutex_init(&c->iterator[i].lock);
+	for (int i = 0; i < NUM_CALL_ITERATORS; i++) {
+		mutex_init(&c->iterator[i].next_lock);
+		mutex_init(&c->iterator[i].prev_lock);
+	}
 
 	return c;
 }
@@ -3150,7 +3154,7 @@ restart:
 				if (rtpe_call_iterators[i].first) {
 					first_call = rtpe_call_iterators[i].first->data;
 					// coverity[lock_order : FALSE]
-					if (mutex_trylock(&first_call->iterator[i].lock)) {
+					if (mutex_trylock(&first_call->iterator[i].prev_lock)) {
 						mutex_unlock(&rtpe_call_iterators[i].lock);
 						continue; // retry
 					}
@@ -3162,7 +3166,7 @@ restart:
 				= g_list_insert_before_link(rtpe_call_iterators[i].first,
 					rtpe_call_iterators[i].first, &c->iterator[i].link);
 			if (first_call)
-				mutex_unlock(&first_call->iterator[i].lock);
+				mutex_unlock(&first_call->iterator[i].prev_lock);
 			mutex_unlock(&rtpe_call_iterators[i].lock);
 		}
 

include/call.h

@@ -427,7 +427,8 @@ struct call_iterator_list {
 };
 struct call_iterator_entry {
 	GList link; // .data is protected by the list's main lock
-	mutex_t lock; // held while the link is in use, protects link.prev and link.next
+	mutex_t next_lock; // held while the link is in use, protects link.data and link.next
+	mutex_t prev_lock; // held while the link is in use, protects link.prev
 };
 
 #define ITERATE_CALL_LIST_START(which, varname) \
@@ -436,15 +437,28 @@ struct call_iterator_entry {
 		mutex_lock(&rtpe_call_iterators[__which].lock); \
 		\
 		GList *__l = rtpe_call_iterators[__which].first; \
+		struct call *next_ ## varname = NULL; \
 		while (__l) { \
-			struct call *varname = __l->data; \
+			struct call *varname = NULL; \
+			if (next_ ## varname) \
+				varname = next_ ## varname; \
+			else { \
+				varname = __l->data; \
 				obj_hold(varname); \
-			mutex_lock(&varname->iterator[__which].lock); \
+				mutex_lock(&varname->iterator[__which].next_lock); \
+			} \
 			mutex_unlock(&rtpe_call_iterators[__which].lock)
 
 #define ITERATE_CALL_LIST_NEXT_END(varname) \
 			GList *__next = varname->iterator[__which].link.next; \
-			mutex_unlock(&varname->iterator[__which].lock); \
+			if (__next) { \
+				next_ ## varname = __next->data; \
+				obj_hold(next_ ## varname); \
+				mutex_lock(&next_ ## varname->iterator[__which].next_lock); \
+			} \
+			else \
+				next_ ## varname = NULL; \
+			mutex_unlock(&varname->iterator[__which].next_lock); \
 			__l = __next; \
 			obj_put(varname); \
 			mutex_lock(&rtpe_call_iterators[__which].lock); \