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Isolation Alloc

Isolation Alloc (or IsoAlloc) is a secure and fast(ish) memory allocator written in C. It is a drop in replacement for malloc on Linux / Mac OS using LD_PRELOAD. Its security strategy is partially inspired by Chrome's PartitionAlloc. A memory allocation isolation security strategy is best summed up as keeping objects of different sizes or types separate from one another. Strict allocation isolation can be achieved using the IsoAlloc APIs directly.

Isolation Alloc is designed and tested for 64 bit Linux. The space afforded by a 64 bit process makes this possible, therefore Isolation Alloc does not support 32 bit targets. It may work in a 32 bit address space but it remains untested and the number of bits of entropy provided to mmap based page allocations is far too low in a 32 bit process to provide much security value. It may work on operating systems other than Linux/Mac OS but that is also untested at this time.

Additional information about the allocator and some of its design choices can be found here.


You can think of Isolation Alloc as a region based memory allocator. If you are familiar with the implementation of arenas in other allocators then the concepts here will be familiar to you.

There is one iso_alloc_root structure which contains a fixed number of iso_alloc_zone structures. These iso_alloc_zone structures are referred to as zones. Zones point to user chunks and a bitmap that is used to manage those chunks. The translation between bitmap and user chunks is referred to as bit slots. Both of these allocations are done separately, the zone only maintains pointers to them. These pointers are masked in between alloc and free operations. The bitmap contains 2 bits of state per user chunk. The current bit value specification is as follows:

  • 00 free chunk
  • 10 currently in use
  • 01 was used but is now free
  • 11 canary chunk

All user chunk pages and bitmap pages are surrounded by guard page allocations with the PROT_NONEpermission. Zones are created for specific sizes, or manually created through the exposed API for a particular size or object type. Internally managed zones will live for the entire lifetime of the process, but zones created via the API can be destroyed at any time.

If DEBUG, LEAK_DETECTOR, or MEM_USAGE are specified during compilation a memory leak and memory usage routine will be called from the destructor which will print useful information about the state of the heap at that time. These can also be invoked via the API, which is documented below.

  • All allocations are 8 byte aligned
  • The iso_alloc_root structure is thread safe and guarded by an atomic lock
  • The bitmap has 2 bits set aside per chunk
  • Zones are 8 MB in size regardless of the chunk sizes they manage
  • Default zones are created in the constructor for sizes: 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192 bytes. Zones are created on demand for larger allocations
  • The free bit slot cache is 255 entries, it helps speed up allocations
  • All allocations >262144 bytes live in specially handled big zones which have no size limitations

There is support for Address Sanitizer, Memory Sanitizer, and Undefined Behavior Sanitizer. If you want to enable it just uncomment the ENABLE_ASAN, ENABLE_MSAN, or ENABLE_UBSAN flags in the Makefile. Like any other usage of Address Sanitizer these are mutually exclusive. IsoAlloc will use Address Sanitizer macros to poison and unpoison user chunks appropriately. IsoAlloc still catches a number of issues Address Sanitizer does not including double/unaligned/wild free's.

A feature similar to GWP-ASAN can be enabled with ALLOC_SANITY in the Makefile. It samples calls to iso_alloc/malloc and allocates a page of memory surrounded by guard pages in order to detect Use-After-Free and linear heap overflows. This feature works on all supported platforms.

You can also enable UNINIT_READ_SANITY for detecting uninitialized read vulnerabilities using the userfaultfd syscall. You can read more about that feature here. This feature is only available on Linux.

See the PERFORMANCE documentation for more information.

Thread Safety

IsoAlloc is thread safe by way of protecting the root structure with an atomic lock built with c11 atomic_flag support. This means every thread that wants to allocate or free a chunk needs to wait until it can grab the lock. This design choice has some big pros and cons. It can negatively impact performance of multi threaded programs that perform a lot of allocations. This is because every thread shares the same set of global zones. The benefit of this is that you can allocate and free any chunk from any thread with little code complexity required. In order to help alleviate contention on this atomic lock each thread has a zone cache built using thread local storage. This is implemented as a simple FILO cache of the most recently used zones by that thread. It's size can be increased using the THREAD_CACHE_SZ define in the internal header file. Making this cache too large can lead to negative performance implications for certain allocation patterns. For example, if a thread allocates multiple 32 byte chunks in a row then the cache may be populated entirely by the same zone that holds 32 byte chunks. Now when the thread goes to allocate a 64 byte chunk it iterates through the entire cache, does not find a usable zone, and then has to take the slow path which iterates through all zones again. You can disable this cache by setting THREAD_ZONE_CACHE to 0 in the Makefile.

When enabled the CPU_PIN feature will restrict allocations from a given zone to the CPU core that created that zone. Free operations are not restricted in this way. This mode is compatible with and without thread support, is only supported on Linux, and will introduce a slight performance hit to the hot path and may increase memory usage. The benefit of this mode is that it introduces an isolation mechanism based on CPU core with no configuration beyond enabling the CPU_PIN define in the Makefile.

Security Properties

  • Zones cannot overflow or underflow into one another
  • All user pages are surrounded by guard pages including big zones
  • All bitmap pages are surrounded by guard pages
  • Double free's are checked for on every call to iso_free
  • For zones managing allocations <= 8192 bytes in size around %1 of their chunks are permanent canaries
  • All free'd chunks get a canary written to them and verified upon reallocation
  • The state of all zones can be verified at any anytime using iso_verify_zones or iso_verify_zone(zone)
  • Canaries are unique and are composed of a 64 bit secret value xor'd by the address of the chunk itself
  • A reused chunk will always have its canary checked before its returned by iso_alloc
  • The top byte of user chunk canaries is 0x00 to prevent unbounded C string reads from leaking it
  • A chunk can be permanently free'd with a call to iso_free_permanently
  • If SANITIZE_CHUNKS is set all user chunks are cleared when passed to iso_free with the constant 0xDE
  • When freeing a chunk the canary in adjacent chunks above/below are verified
  • Some important zone metadata pointers are masked inbetween iso_alloc and iso_free operations
  • Passing a pointer to iso_free that was not allocated with iso_alloc will abort
  • Pointers passed to iso_free must be 8 byte aligned, and a multiple of the zone chunk size
  • The free bit slot cache provides a chunk quarantine or delayed free mechanism
  • The free bit slot cache is checked for duplicate entries to detect corruption
  • When custom zones are destroyed they are overwritten and marked PROT_NONE to prevent use-after-free
  • Big zone meta data lives at a random offset from its base page
  • A call to realloc will always return a new chunk. Use PERM_FREE_REALLOC to make these free's permanent
  • Enable FUZZ_MODE in the Makefile to verify all zones upon alloc/free, and never reuse custom zones
  • When CPU_PIN is enabled allocation from a zone will be restricted to the CPU core that created it


The Makefile targets are very simple:

make library - Builds a release version of the library without C++ support

make library_debug - Builds a debug version of the library

make library_debug_no_output - Builds a debug version of the library with no logging output

make analyze_library_debug - Builds the library with clang's scan-build if installed

make tests - Builds and runs all tests

make perf_tests - Builds and runs a simple performance test that uses gprof. Linux only

make malloc_cmp_test - Builds and runs a test that uses both iso_alloc and malloc for comparison

make c_library_objects - Builds .o files to be linked in another compilation step

make c_library_objects_debug - Builds debug .o files to be linked in another compilation step

make cpp_library - Builds the library with a C++ interface that overloads operators new and delete

make cpp_library_debug - Builds a debug version of the library with a C++ interface that overloads operators new and delete

make cpp_tests - Builds and runs the C++ tests

make format - Runs clang formatter according to the specification in .clang-format

make clean - Cleans up the root directory

Linking With C++

If you want to use IsoAlloc with a C++ program you can use the c_library_objects Makefile target. This will produce .o object files you can pass to your compiler. These targets are used internally to build a library with new and delete support.


If you try to use Isolation Alloc in an existing program then and you are getting crashes here are some tips to help you get started. First make sure you actually replaced all malloc, calloc, realloc and free calls to their iso_alloc equivalents. Don't forget things like strdup that return a pointer from malloc.

If you are getting consistent crashes you can build a debug version of the library with make library_debug and then catch the crash in GDB with a command similar to this gdb -q -command=misc/commands.gdb <your_binary>.

If all else fails please file an issue on the github project page.


void *iso_alloc(size_t size) - Equivalent to malloc. Returns a pointer to a chunk of memory that is size bytes in size. To free this chunk just pass it to iso_free.

void *iso_calloc(size_t nmemb, size_t size) - Equivalent to calloc. Allocates a chunk big enough for an array of nmemb elements of size bytes. The array is zeroized.

void *iso_realloc(void *p, size_t size) - Equivalent to realloc. Reallocates a new chunk, if necessary, to be size bytes big and copies the contents of p to it.

void iso_free(void *p) - Frees any chunk allocated and returned by any API call (e.g. iso_alloc, iso_calloc, iso_realloc, iso_strdup, iso_strndup).

void iso_free_permanently(void *p) - Same as iso_free but marks the chunk in such a way that it will not be reallocated

size_t iso_chunksz(void *p) - Returns the size of the chunk returned by iso_alloc

char *iso_strdup(const char *str) - Equivalent to strdup. Returned pointer must be free'd by iso_free.

char *iso_strndup(const char *str, size_t n) - Equivalent to strndup. Returned pointer must be free'd by iso_free.

iso_alloc_zone_handle *iso_alloc_new_zone(size_t size) - Allocates a new private zone for allocations up to size bytes. Returns a handle to that zone.

char *iso_strdup_from_zone(iso_alloc_zone_handle *zone, const char *str) - Equivalent to iso_strdup except string is duplicated in specified zone.

char *iso_strndup_from_zone(iso_alloc_zone_handle *zone, const char *str, size_t n) - Equivalent to iso_strndup except string is duplicated in specified zone.

iso_alloc_zone_handle *iso_alloc_from_zone(iso_alloc_zone_handle *zone, size_t size) - Equivalent to iso_alloc except allocation is done in specified zone.

void iso_alloc_destroy_zone(iso_alloc_zone_handle *zone) - Destroy a zone created with iso_alloc_from_zone.

void iso_alloc_protect_root() - Temporarily protects the iso_alloc root structure by marking it unreadable.

void iso_alloc_unprotect_root() - Undoes the operation performed by iso_alloc_protect_root.

uint64_t iso_alloc_detect_leaks() - Returns the total number of leaks detected for all zones. Will print debug logs when compiled with -DDEBUG

uint64_t iso_alloc_detect_zone_leaks(iso_alloc_zone_handle *zone) - Returns the total number of leaks detected for specified zone. Will print debug logs when compiled with -DDEBUG

uint64_t iso_alloc_mem_usage() - Returns the total memory usage for all zones. Will print debug logs when compiled with -DDEBUG

uint64_t iso_alloc_zone_mem_usage(iso_alloc_zone_handle *zone) - Returns the total memory usage for a specified zone. Will print debug logs when compiled with -DDEBUG

void iso_verify_zones() - Verifies the state of all zones. Will abort if inconsistencies are found.

void iso_verify_zone(iso_alloc_zone_handle *zone) - Verifies the state of specified zone. Will abort if inconsistencies are found.

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