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https://github.com/jedisct1/was-not-wasm
A hostile memory allocator to make WebAssembly applications more predictable.
https://github.com/jedisct1/was-not-wasm
assemblyscript heartbleed malloc memory-management security wasm webassembly
Last synced: 3 months ago
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A hostile memory allocator to make WebAssembly applications more predictable.
- Host: GitHub
- URL: https://github.com/jedisct1/was-not-wasm
- Owner: jedisct1
- Created: 2019-02-24T22:46:43.000Z (almost 6 years ago)
- Default Branch: master
- Last Pushed: 2022-05-04T04:12:16.000Z (over 2 years ago)
- Last Synced: 2024-10-05T04:34:06.204Z (3 months ago)
- Topics: assemblyscript, heartbleed, malloc, memory-management, security, wasm, webassembly
- Language: Rust
- Homepage:
- Size: 26.4 KB
- Stars: 82
- Watchers: 3
- Forks: 1
- Open Issues: 2
-
Metadata Files:
- Readme: README.md
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README
# WAS (not WASM)
A hostile memory allocator to make WebAssembly applications more
predictable.
## Blurb
The WebAssembly memory model
[doesn't offer any protection](https://00f.net/2018/11/25/webassembly-doesnt-make-unsafe-languages-safe/)
against buffer underflows/overflows.
As long as accesses are made within the bounds of the linear memory
segments, no page faults will ever occur.
Besides facilitating heartbleed-class vulnerabilities, this memory
model is also painful for application developers.
Where a native application may crash in the same context,
out-of-bounds accesses in a WebAssembly application may cause silent
memory corruption and subtle, tedious-to-debug bugs.
WAS (not WASM) is a simple memory allocator designed to catch memory
issues in WebAssembly compilers and applications.
WAS (not WASM) makes the heap inaccessible, except for pages
explicitly allocated by the application.
WAS (not WASM) makes static data read-only. Writing to a `NULL`
pointer will fault.
WAS (not WASM) never reuses allocated pages after they are `free()`'d.
Deallocated pages become inaccessible.
WAS (not WASM) fills newly allocated regions with junk.
WAS (not WASM) ensures that a guard page immediately follows every
single allocation, so that a single-byte overflow will cause a fault.
WAS (not WASM) inserts a canary in partially allocated pages, and
verifies that it hasn't been tampered with in order to detected underflows.
WAS (not WASM) detects double-free(), use-after-free(), invalid free().
WAS (not WASM) keeps track of the number of allocations, deallocations
and total memory usage, so you can scream at how many of these
WebAssembly applications do, and optimize yours accordingly.
WAS (not WASM) is not designed to be fast. It is designed to help you
develop safer applications. Or eventually faster applications, by using
unsafe constructions with more confidence.
WAS (not WASM) runs WASM code using
[Cranelift](https://github.com/CraneStation/cranelift)
and [Wasmer](https://wasmer.io/).
## Installation
Install Rust, and use `cargo`:
```sh
cargo install
```
## Usage
```text
USAGE:
was [FLAGS] [OPTIONS] --file
FLAGS:
-c, --canary-check-on-alloc
-h, --help Prints help information
-V, --version Prints version information
OPTIONS:
-e, --entrypoint [default: main]
-f, --file
-b, --heap-base [default: 65536]
```
Example:
```sh
was -f app.wasm
```
The `--canary-check-on-alloc` option checks every single canary before
every single allocation. This is slow, and will get slower as the
number of allocation grows.
The `--heap-base` option sets how much data is already present on the
heap before dynamic allocations are performed. This is typically used
to store static data. When using AssemblyScript, the optimal value for
the heap base is stored in the `HEAP_BASE` global.
## Usage with AssemblyScript
WAS (not WASM) was originally made to work with [AssemblyScript](https://assemblyscript.org).
In order to do so, use the `system` allocator:
```typescript
import 'allocator/system';
```
Optionally, in order to check canaries when the application
terminates, call the `terminate()` function in your `index.ts` file:
```typescript
declare function terminate(): void;
@global export function main(): void {
...
terminate();
}
```
AssemblyScript stores static data at the beginning of the heap. The
heap base after this static data is stored in the `HEAP_BASE` global.
A quick way to print it while using WAS (not WASM) is to temporarily
add this to your application:
```typescript
declare function debug_val(val: u32): void;
debug_val(HEAP_BASE);
```