document NEVER_REUSE_ZONES and make custom zones unusable when destroyed

Author: struct

Makefile

@@ -9,7 +9,9 @@ CXX = clang++
 ## FUZZ_MODE - Call verify_all_zones upon alloc/free, never reuse custom zones
 ## PERM_FREE_REALLOC - Permanently free any realloc'd chunk
 ## DISABLE_CANARY - Disables the use of canaries, improves performance
-SECURITY_FLAGS = -DSANITIZE_CHUNKS=0 -DFUZZ_MODE=0 -DPERM_FREE_REALLOC=0 -DDISABLE_CANARY=0
+## NEVER_REUSE_ZONES - Tells IsoAlloc to unmap user and bitmap pages when destroying custom zones
+SECURITY_FLAGS = -DSANITIZE_CHUNKS=0 -DFUZZ_MODE=0 -DPERM_FREE_REALLOC=0 -DDISABLE_CANARY=0	\
+                 -DNEVER_REUSE_ZONES=0
 
 ## Enable abort() when isoalloc can't gather enough entropy.
 ABORT_NO_ENTROPY = -DABORT_NO_ENTROPY=1

README.md

@@ -82,6 +82,7 @@ When enabled, the `CPU_PIN` feature will restrict allocations from a given zone
 * By default `NO_ZERO_ALLOCATIONS` will return a pointer to a page marked `PROT_NONE` for all `0` sized allocations.
 * When `ABORT_NO_ENTROPY` is enabled IsoAlloc will abort when it can't gather enough entropy.
 * When `SHUFFLE_BIT_SLOT_CACHE` is enabled IsoAlloc will shuffle the bit slot cache upon creation (3-4x perf hit)
+* When destroying custom zones if `NEVER_REUSE_ZONES` is enabled IsoAlloc won't attempt to repurpose the zone
 
 ## Building
 

src/iso_alloc.c

@@ -463,11 +463,16 @@ INTERNAL_HIDDEN void _iso_alloc_destroy_zone(iso_alloc_zone *zone) {
 
     if(zone->internally_managed == false) {
 #if NEVER_REUSE_ZONES || FUZZ_MODE
-        _unmap_zone(zone);
-        zone->user_pages_start = NULL;
-        zone->bitmap_start = NULL;
+        memset(zone->bitmap_start, 0x0, zone->bitmap_size);
+        memset(zone->user_pages_start, 0x0, ZONE_USER_SIZE);
+
+        /* This will waste memory because we will never unmap
+         * these pages, even in the destructor */
+        mprotect_pages(zone->bitmap_start, zone->bitmap_size, PROT_NONE);
+        mprotect_pages(zone->user_pages_start, ZONE_USER_SIZE, PROT_NONE);
 
-        /* Mark the zone as full so no attempts are made to use it */
+        /* Make this zone unusable */
+        memset(zone, 0x0, sizeof(iso_alloc_zone));
         zone->is_full = true;
         flush_thread_zone_cache();
 #else