--- a/lib/util/secport.c
+++ b/lib/util/secport.c
@@ -64,50 +64,63 @@ unsigned long port_allocFailures;
 /* locations for registering Unicode conversion functions.  
  * XXX is this the appropriate location?  or should they be
  *     moved to client/server specific locations?
  */
 PORTCharConversionFunc ucs4Utf8ConvertFunc;
 PORTCharConversionFunc ucs2Utf8ConvertFunc;
 PORTCharConversionWSwapFunc  ucs2AsciiConvertFunc;
 
+/* NSPR memory allocation functions (PR_Malloc, PR_Calloc, and PR_Realloc)
+ * use the PRUint32 type for the size parameter. Before we pass a size_t or
+ * unsigned long size to these functions, we need to ensure it is <= half of
+ * the maximum PRUint32 value to avoid truncation and catch a negative size.
+ */
+#define MAX_SIZE (PR_UINT32_MAX >> 1)
+
 void *
 PORT_Alloc(size_t bytes)
 {
-    void *rv;
+    void *rv = NULL;
 
-    /* Always allocate a non-zero amount of bytes */
-    rv = (void *)PR_Malloc(bytes ? bytes : 1);
+    if (bytes <= MAX_SIZE) {
+	/* Always allocate a non-zero amount of bytes */
+	rv = PR_Malloc(bytes ? bytes : 1);
+    }
     if (!rv) {
 	++port_allocFailures;
 	PORT_SetError(SEC_ERROR_NO_MEMORY);
     }
     return rv;
 }
 
 void *
 PORT_Realloc(void *oldptr, size_t bytes)
 {
-    void *rv;
+    void *rv = NULL;
 
-    rv = (void *)PR_Realloc(oldptr, bytes);
+    if (bytes <= MAX_SIZE) {
+	rv = PR_Realloc(oldptr, bytes);
+    }
     if (!rv) {
 	++port_allocFailures;
 	PORT_SetError(SEC_ERROR_NO_MEMORY);
     }
     return rv;
 }
 
 void *
 PORT_ZAlloc(size_t bytes)
 {
-    void *rv;
+    void *rv = NULL;
 
-    /* Always allocate a non-zero amount of bytes */
-    rv = (void *)PR_Calloc(1, bytes ? bytes : 1);
+    if (bytes <= MAX_SIZE) {
+	/* Always allocate a non-zero amount of bytes */
+	rv = PR_Calloc(1, bytes ? bytes : 1);
+    }
     if (!rv) {
 	++port_allocFailures;
 	PORT_SetError(SEC_ERROR_NO_MEMORY);
     }
     return rv;
 }
 
 void
@@ -204,33 +217,35 @@ PORT_GetError(void)
  * until the arenapool is destroyed, at which point all the arenas are
  * freed or returned to a "free arena list", depending on their sizes.
  */
 PLArenaPool *
 PORT_NewArena(unsigned long chunksize)
 {
     PORTArenaPool *pool;
     
+    if (chunksize > MAX_SIZE) {
+	PORT_SetError(SEC_ERROR_NO_MEMORY);
+	return NULL;
+    }
     pool = PORT_ZNew(PORTArenaPool);
     if (!pool) {
 	return NULL;
     }
     pool->magic = ARENAPOOL_MAGIC;
     pool->lock = PZ_NewLock(nssILockArena);
     if (!pool->lock) {
 	++port_allocFailures;
 	PORT_Free(pool);
 	return NULL;
     }
     PL_InitArenaPool(&pool->arena, "security", chunksize, sizeof(double));
     return(&pool->arena);
 }
 
-#define MAX_SIZE 0x7fffffffUL
-
 void *
 PORT_ArenaAlloc(PLArenaPool *arena, size_t size)
 {
     void *p = NULL;
 
     PORTArenaPool *pool = (PORTArenaPool *)arena;
 
     if (size <= 0) {
@@ -325,16 +340,21 @@ PORT_FreeArena(PLArenaPool *arena, PRBoo
 }
 
 void *
 PORT_ArenaGrow(PLArenaPool *arena, void *ptr, size_t oldsize, size_t newsize)
 {
     PORTArenaPool *pool = (PORTArenaPool *)arena;
     PORT_Assert(newsize >= oldsize);
     
+    if (newsize > MAX_SIZE) {
+	PORT_SetError(SEC_ERROR_NO_MEMORY);
+	return NULL;
+    }
+
     if (ARENAPOOL_MAGIC == pool->magic ) {
 	PZ_Lock(pool->lock);
 	/* Do we do a THREADMARK check here? */
 	PL_ARENA_GROW(ptr, arena, oldsize, ( newsize - oldsize ) );
 	PZ_Unlock(pool->lock);
     } else {
 	PL_ARENA_GROW(ptr, arena, oldsize, ( newsize - oldsize ) );
     }