Skip to main content

Runtime Extensions

Memory Management

Whenever a computer program requires memory to store data (e.g., some text, an image, etc. ), it must allocate memory. Allocating memory is the mechanism by which the operating system grants some of its storage and processing power to a running program to ensure that the problem can run properly.

In this context, there is also what is commonly known as "virtual memory space". Every program (process) running does not need to know the actual address in the memory but only an abstracted version.

The Stronghold runtime provides memory (speak managed allocation) types to protect sensitive data.

Boxed Memory

Boxed memory locks allocated memory and prevents it from being recorded in a memory dump. Since locking memory is dependent on the operating system, the Boxed type relies on Libsodium’s sodium_mlock function. This function calls the mlock function on Linux (or equivalent functions on other operating systems). mlock prevents the current virtual address space of the process from being paged into a swap area, preventing the leakage of sensitive data. This, in turn, will be used by guarded heap allocations of memory.

Guarded heap allocations work by placing a guard page in front and at the end of the locked memory and a canary value at the front. The schematic view visualizes it.

Guarded Heap Memory Allocations

Libsodium provides three types to guard memory:

sodium_mprotect_noaccessMakes the protected memory inaccessible. It can neither be read from nor written to.
sodium_mprotect_readonlyMakes the protected memory read-only. Memory can be read from but not written to.
sodium_mprotect_readwriteEnables reading from and writing to protected memory.

Stronghold exposes locked memory via the LockedMemory trait, that exposes two functions that need to be implemented:

/// Modifies the value and potentially reallocates the data.
fn update(self, payload: Buffer<u8>, size: usize) -> Result<Self, MemoryError>;

/// Unlocks the memory and returns a Buffer
fn unlock(&self) -> Result<Buffer<u8>, MemoryError>;

Currently, three types of memory implement this trait:

RamMemoryAllocated values reside inside the system's ram.
FileMemoryAllocated values reside on the file system
NonContiguousMemoryAllocated memory is fragmented across the system's ram or file system, or a combination of both.