The Rust community is currently discussing the recently released mem-isolate library, which promises to run unsafe code safely through a technique that isolates potentially dangerous operations in forked processes. While the approach is clever, security and systems programming experts have raised significant concerns about its limitations and potential misuse.
mem-isolate works by executing functions in isolated child processes created via the POSIX fork() system call. This approach creates a copy of the parent process's memory, allowing unsafe operations to run without affecting the original process's memory space. When the function completes, results are serialized back to the parent process through a pipe, and the child process terminates.
Security Limitations
Security experts in the community have pointed out that mem-isolate doesn't actually provide the level of safety implied by its name. The library doesn't meet Rust's definition of safe code, which requires both spatial and temporal memory safety guarantees.
I don't think this meets the definition of safe in safe Rust: safe doesn't just mean won't crash due to spatial memory errors, it means that the code is in fact spatially and temporally memory safe.
From a security perspective, the isolation provided is superficial. The forked process maintains a complete copy of program state, including any secrets in memory. This means that exploitable unsafe code could still access sensitive information within the isolated environment. Additionally, the child process maintains the same privileges as the parent, so code execution vulnerabilities remain exploitable.
Key Limitations of mem-isolate:
- Only works on POSIX systems (Linux, macOS, BSD)
- Introduces ~1.9ms overhead per function call (compared to 1.5ns for direct calls)
- Requires serialization of returned data between processes
- Potentially dangerous in multi-threaded applications
- Does not prevent exploits that don't cause crashes
- Child process has same privileges and access to memory as parent
- Can cause deadlocks if forking while mutexes are held
Technical Concerns with Fork
Systems programming experts have highlighted that fork() is a problematic API for this use case, especially in multi-threaded applications. When a process forks, the entire memory state is replicated, including the state of mutexes and locks. If any thread was holding a lock at the time of forking, the child process could experience deadlocks.
Even simple operations like printing or memory allocation can cause freezes in the forked process. User-space buffers that aren't flushed before the callback completes will be lost. These issues make mem-isolate potentially dangerous in complex applications.
Performance Implications
The library introduces significant performance overhead, with benchmarks showing that execute_in_isolated_process() takes approximately 1.9ms compared to 1.5ns for a direct function call – over a million times slower. While the author acknowledges this limitation with a tongue-in-cheek remark about running code 1ms slower, community members point out that this overhead makes the library impractical for many use cases.
Many developers use unsafe code specifically for performance optimization, so wrapping such code in a mechanism that introduces substantial overhead defeats the original purpose. As one commenter noted, using this approach extensively could negate Rust's performance advantages over languages like Python or Ruby that rely heavily on forking for parallelism.
Performance Benchmarks:
- Direct function call: ~1.5ns
- fork() + wait: ~1.7ms
- execute_in_isolated_process(): ~1.9ms
Practical Applications
Despite its limitations, some developers see value in mem-isolate for specific scenarios. The library could be useful when working with inherently unsafe third-party code that cannot be improved, particularly when interfacing with C libraries. In these cases, the isolation provided might be an acceptable trade-off for added safety, especially in non-performance-critical paths.
For most applications, however, experts recommend more robust approaches to isolation. Properly designed multi-process architectures with restricted IPC channels and OS-level sandboxing APIs provide stronger security guarantees. Browser engines like Chrome use this approach rather than simple process isolation.
The discussion around mem-isolate highlights the ongoing challenges in balancing safety, performance, and practicality in systems programming. While innovative approaches to memory safety are welcome in the Rust ecosystem, they must be evaluated carefully against established best practices and security models.
Reference: mem-isolate: Run unsafe code safely