From 7b65180b32558fc4f2bc7b6ac5602f306ed3a014 Mon Sep 17 00:00:00 2001
From: Jason Evans <jasone@canonware.com>
Date: Sun, 20 Oct 2013 14:09:54 -0700
Subject: [PATCH] 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.
---
 ChangeLog |  2 ++
 src/ctl.c | 84 ++++++++++++++++++++++++++++++-------------------------
 2 files changed, 48 insertions(+), 38 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 9b51fd57..def7685e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -9,6 +9,8 @@ found in the git revision history:
 * 3.4.1 (XXX)
 
   Bug fixes:
+  - Fix a race in the "arenas.extend" mallctl that could cause memory corruption
+    of internal data structures and subsequent crashes.
   - Fix Valgrind integration flaws that caused Valgrind warnings about reads of
     uninitialized memory in:
     + arena chunk headers
diff --git a/src/ctl.c b/src/ctl.c
index f278105a..ebba7c25 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);
-	}
-	/* Initialize the new astats and arenas elements. */
-	memset(&astats[ctl_stats.narenas + 1], 0, sizeof(ctl_arena_stats_t));
-	if (ctl_arena_init(&astats[ctl_stats.narenas + 1]))
+	/* 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[ctl_stats.narenas] = NULL;
+	tarenas = (arena_t **)imalloc((ctl_stats.narenas + 1) *
+	    sizeof(arena_t *));
+	if (tarenas == NULL) {
+		idalloc(astats);
+		return (true);
+	}
+
+	/* 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])) {
+		idalloc(tarenas);
+		idalloc(astats);
+		return (true);
+	}
 	/* 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);
 }