Isolation Alloc is a secure and fast(ish) memory allocator written in C. 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. 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 (it currently compiles on 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.
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
- Zones are thread safe by default by a mutex
- The bitmap has 2 bits set aside per chunk
- All 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
- Zones cannot overflow or underflow into one another
- All user pages are surrounded by guard pages
- All bitmap pages are surrounded by guard pages
- Double free's are checked for on every call to
- For zones managing allocations <= 8192 bytes in size around %1 of their chunks are canaries
- The state of all zones can be verified at any anytime using
- A reused chunk will always have its canary checked before its returned by
- A chunk can be permanently free'd with a call to
- All user chunk contents are cleared when passed to
iso_freewith the constant 0xDE
- When freeing a chunk the canary in adjacent chunks above/below are verified
- Some important zone metadata pointers are masked inbetween
- Passing a pointer to
iso_freethat was not allocated with
- Pointers passed to
iso_freemust be 8 byte aligned, and a multiple of the zone chunk size
- 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
The Makefile targets are very simple:
make library - Builds a standard version of the library
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 tests - Builds and runs all tests
make perf_tests - Builds and run a simple performance test
make cpp_library - Builds the library with a simple C++ interface that overloads new/delete operators
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
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
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
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
char *iso_strdup(const char *str) - Equivalent to strdup. Returned pointer must be free'd by
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 reallocation is done in specified zone.
iso_alloc_zone_handle *iso_realloc_from_zone(iso_alloc_zone_handle *zone, void *p, size_t size) - Equivalent to iso_realloc except reallocation is done in specified zone.
void iso_alloc_destroy_zone(iso_alloc_zone_handle *zone) - Destroy a zone created with
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
int64_t iso_alloc_detect_leaks() - Returns the total number of leaks detected for all zones. Will print debug logs when compiled with
int64_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
int64_t iso_alloc_mem_usage() - Returns the total memory usage for all zones. Will print debug logs when compiled with
int64_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
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.