Fix a race condition in the "arenas.extend" mallctl.

Fix a race condition in the "arenas.extend" mallctl that could lead to
internal data structure corruption.  The race could be hit if one
thread called the "arenas.extend" mallctl while another thread
concurrently triggered initialization of one of the lazily created
arenas.

1 files changed, 44 insertions, 36 deletions

diff --git a/src/ctl.c b/src/ctl.c
index f278105..ebba7c2 100644
--- a/src/ctl.c
+++ b/src/ctl.c
@@ -546,43 +546,30 @@ ctl_arena_refresh(arena_t *arena, unsigned i)
 static bool
 ctl_grow(void)
 {
-	size_t astats_size;
 	ctl_arena_stats_t *astats;
 	arena_t **tarenas;
 
-	/* Extend arena stats and arenas arrays. */
-	astats_size = (ctl_stats.narenas + 2) * sizeof(ctl_arena_stats_t);
-	if (ctl_stats.narenas == narenas_auto) {
-		/* ctl_stats.arenas and arenas came from base_alloc(). */
-		astats = (ctl_arena_stats_t *)imalloc(astats_size);
-		if (astats == NULL)
-			return (true);
-		memcpy(astats, ctl_stats.arenas, (ctl_stats.narenas + 1) *
-		    sizeof(ctl_arena_stats_t));
-
-		tarenas = (arena_t **)imalloc((ctl_stats.narenas + 1) *
-		    sizeof(arena_t *));
-		if (tarenas == NULL) {
-			idalloc(astats);
-			return (true);
-		}
-		memcpy(tarenas, arenas, ctl_stats.narenas * sizeof(arena_t *));
-	} else {
-		astats = (ctl_arena_stats_t *)iralloc(ctl_stats.arenas,
-		    astats_size, 0, 0, false, false);
-		if (astats == NULL)
-			return (true);
-
-		tarenas = (arena_t **)iralloc(arenas, (ctl_stats.narenas + 1) *
-		    sizeof(arena_t *), 0, 0, false, false);
-		if (tarenas == NULL)
-			return (true);
+	/* Allocate extended arena stats and arenas arrays. */
+	astats = (ctl_arena_stats_t *)imalloc((ctl_stats.narenas + 2) *
+	    sizeof(ctl_arena_stats_t));
+	if (astats == NULL)
+		return (true);
+	tarenas = (arena_t **)imalloc((ctl_stats.narenas + 1) *
+	    sizeof(arena_t *));
+	if (tarenas == NULL) {
+		idalloc(astats);
+		return (true);
 	}
-	/* Initialize the new astats and arenas elements. */
+
+	/* Initialize the new astats element. */
+	memcpy(astats, ctl_stats.arenas, (ctl_stats.narenas + 1) *
+	    sizeof(ctl_arena_stats_t));
 	memset(&astats[ctl_stats.narenas + 1], 0, sizeof(ctl_arena_stats_t));
-	if (ctl_arena_init(&astats[ctl_stats.narenas + 1]))
+	if (ctl_arena_init(&astats[ctl_stats.narenas + 1])) {
+		idalloc(tarenas);
+		idalloc(astats);
 		return (true);
-	tarenas[ctl_stats.narenas] = NULL;
+	}
 	/* Swap merged stats to their new location. */
 	{
 		ctl_arena_stats_t tstats;
@@ -593,13 +580,34 @@ ctl_grow(void)
 		memcpy(&astats[ctl_stats.narenas + 1], &tstats,
 		    sizeof(ctl_arena_stats_t));
 	}
+	/* Initialize the new arenas element. */
+	tarenas[ctl_stats.narenas] = NULL;
+	{
+		arena_t **arenas_old = arenas;
+		/*
+		 * Swap extended arenas array into place.  Although ctl_mtx
+		 * protects this function from other threads extending the
+		 * array, it does not protect from other threads mutating it
+		 * (i.e. initializing arenas and setting array elements to
+		 * point to them).  Therefore, array copying must happen under
+		 * the protection of arenas_lock.
+		 */
+		malloc_mutex_lock(&arenas_lock);
+		arenas = tarenas;
+		memcpy(arenas, arenas_old, ctl_stats.narenas *
+		    sizeof(arena_t *));
+		narenas_total++;
+		arenas_extend(narenas_total - 1);
+		malloc_mutex_unlock(&arenas_lock);
+		/*
+		 * Deallocate arenas_old only if it came from imalloc() (not
+		 * base_alloc()).
+		 */
+		if (ctl_stats.narenas != narenas_auto)
+			idalloc(arenas_old);
+	}
 	ctl_stats.arenas = astats;
 	ctl_stats.narenas++;
-	malloc_mutex_lock(&arenas_lock);
-	arenas = tarenas;
-	narenas_total++;
-	arenas_extend(narenas_total - 1);
-	malloc_mutex_unlock(&arenas_lock);
 
 	return (false);
 }