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I'll speak to JVM; I'm less familiar with CLR but I believe it's the same. JVM and WASM both have statically-verifiable control flow. No wild jumps, no executa
by ridiculous_fish 7y ago
I'll speak to JVM; I'm less familiar with CLR but I believe it's the same.
JVM and WASM both have statically-verifiable control flow. No wild jumps, no executable stacks, etc. Phew.
Arrays and pointer arithmetic are a big difference. WASM has a big linear memory block, and instructions may access arbitrary locations within it - the runtime performs bounds checking only at the edges. So your `sprintf` can still overflow the buffer, and smash the stack's data, but can't affect the host, or the control flow.
JVM goes further: it prohibits pointer arithmetic and pushes array accesses down into the instruction stream. To access a JVM array, you must provide the array reference itself, and the runtime will perform bounds checking using the length.
The JVM approach gives you better runtime safety - no Heartbleed! The WASM approach is lower-level and is more easily adapted to existing system languages: C++, Rust, other LLVMites.
- pjmlp 7y agoCLR was designed to support C++ since day one. And while WASM trumps the security trumpet, without actually supporting proper bounds checking, the CLR will taint C++ pointer arithmetic as unsafe, thus making the whole module unsafe. So I as consumer can decide if I am willing to trust an unsafe module or not.
- johncolanduoni 7y agoThe CLR doesn’t guarantee control flow integrity (modulo type confusion) or any form of isolation when linear memory accesses are used. So here WASM offers another option in the middle between trust and don’t trust: “trust unsafe module only to not compromise its own functionality; no attacking the rest of the process or kernel” (modulo runtime bugs).
- pjmlp 7y agoWell for that execution of unsafe Assemblies was already enabled anyway, so there isn't much that the verifier can do. Which is something that WASM isn't being honest about, corruption of internal data structures is allowed. If I can control what goes into memory just by calling module public functions with the right data set and access patterns, CFI won't help a thing. Suddenly the authorization module that would authenticate me as regular user, might give me another set of capabilities and off to the races.
- my123 7y agoNote for C++ on the CLR that you can use /clr:safe as an MSVC compilation argument. This errors out when trying to access to random pointers at compile time. /clr:pure uses unsafe and supports those cases though. And yeah, WebAssembly only doing bounds checking within a single memory block and not actually offering true bounds checking is a big downgrade, and a pretty much unjustified one (+ it's rare among JITted languages...).
- zozbot234 7y agoIf you care about "true" bounds checking, just compile to Wasm from a safe source language. Besides, Wasm does support multiple memory blocks so a potentially-unsafe module need not "taint" anything else.