Question

The security model of WebAssembly has two important goals: (1) protect users from buggy or malicious modules, and (2) provide developers with useful primitives and mitigations for developing safe applications, within the constraints of (1).

Users

Each WebAssembly module executes within a sandboxed environment separated from the host runtime using fault isolation techniques. This implies:

• Applications execute independently, and can’t escape the sandbox without going through appropriate APIs.
• Applications generally execute deterministically with limited exceptions.

Additionally, each module is subject to the security policies of its embedding. Within a web browser, this includes restrictions on information flow through same-origin policy. On a non-web platform, this could include the POSIX security model.

Developers

The design of WebAssembly promotes safe programs by eliminating dangerous features from its execution semantics, while maintaining compatibility with programs written for C/C++.

Modules must declare all accessible functions and their associated types at load time, even when dynamic linking is used. This allows implicit enforcement of control-flow integrity (CFI) through structured control-flow. Since compiled code is immutable and not observable at runtime, WebAssembly programs are protected from control flow hijacking attacks.

• Function calls must specify the index of a target that corresponds to a valid entry in the function index space or table index space.
• Indirect function calls are subject to a type signature check at runtime; the type signature of the selected indirect function must match the type signature specified at the call site.
• A protected call stack that is invulnerable to buffer overflows in the module heap ensures safe function returns.
• Branches must point to valid destinations within the enclosing function.

Variables in C/C++ can be lowered to two different primitives in WebAssembly, depending on their scope. Local variables with fixed scope and global variables are represented as fixed-type values stored by index. The former are initialized to zero by default and are stored in the protected call stack, whereas the latter are located in the global index space and can be imported from external modules. Local variables with unclear static scope (e.g. are used by the address-of operator, or are of type struct and returned by value) are stored in a separate user-addressable stack in linear memory at compile time. This is an isolated memory region with fixed maximum size that is zero initialized by default. References to this memory are computed with infinite precision to avoid wrapping and simplify bounds checking. In the future, support for multiple linear memory sections and finer-grained memory operations (e.g. shared memory, page protection, large pages, etc.) will be implemented.

Traps are used to immediately terminate execution and signal abnormal behavior to the execution environment. In a browser, this is represented as a JavaScript exception. Support for module-defined trap handlers will be implemented in the future. Operations that can trap include:

• specifying an invalid index in any index space,
• performing an indirect function call with a mismatched signature,
• exceeding the maximum size of the protected call stack,
• accessing out-of-bounds addresses in linear memory,
• executing an illegal arithmetic operations (e.g. division or remainder by zero, signed division overflow, etc).

Memory Safety

Compared to traditional C/C++ programs, these semantics obviate certain classes of memory safety bugs in WebAssembly. Buffer overflows, which occur when data exceeds the boundaries of an object and accesses adjacent memory regions, cannot affect local or global variables stored in index space, they are fixed-size and addressed by index. Data stored in linear memory can overwrite adjacent objects, since bounds checking is performed at linear memory region granularity and is not context-sensitive. However, the presence of control-flow integrity and protected call stacks prevents direct code injection attacks. Thus, common mitigations such as data execution prevention (DEP) and stack smashing protection (SSP) are not needed by WebAssembly programs.

Another common class of memory safety errors involves unsafe pointer usage and undefined behavior. This includes dereferencing pointers to unallocated memory (e.g. NULL), or freed memory allocations. In WebAssembly, the semantics of pointers have been eliminated for function calls and variables with fixed static scope, allowing references to invalid indexes in any index space to trigger a validation error at load time, or at worst a trap at runtime. Accesses to linear memory are bounds-checked at the region level, potentially resulting in a trap at runtime. These memory region(s) are isolated from the internal memory of the runtime, and are set to zero by default unless otherwise initialized.

Nevertheless, other classes of bugs are not obviated by the semantics of WebAssembly. Although attackers cannot perform direct code injection attacks, it is possible to hijack the control flow of a module using code reuse attacks against indirect calls. However, conventional return-oriented programming (ROP) attacks using short sequences of instructions (“gadgets”) are not possible in WebAssembly, because control-flow integrity ensures that call targets are valid functions declared at load time. Likewise, race conditions, such as time of check to time of use (TOCTOU) vulnerabilities, are possible in WebAssembly, since no execution or scheduling guarantees are provided beyond in-order execution and post-MVP atomic memory primitives :cyclone:. Similarly, side channel attacks can occur, such as timing attacks against modules. In the future, additional protections may be provided by runtimes or the toolchain, such as code diversification or memory randomization (similar to address space layout randomization (ASLR)), or bounded pointers (“fat” pointers).

Control-Flow Integrity

The effectiveness of control-flow integrity can be measured based on its completeness. Generally, there are three types of external control-flow transitions that need to be protected, because the callee may not be trusted:

1. Direct function calls,
2. Indirect function calls,
3. Returns.

Together, (1) and (2) are commonly referred to as “forward-edge”, since they correspond to forward edges in a directed control-flow graph. Likewise (3) is commonly referred to as “back-edge”, since it corresponds to back edges in a directed control-flow graph. More specialized function calls, such as tail calls, can be viewed as a combination of (1) and (3).

Typically, this is implemented using runtime instrumentation. During compilation, the compiler generates an expected control flow graph of program execution, and inserts runtime instrumentation at each call site to verify that the transition is safe. Sets of expected call targets are constructed from the set of all possible call targets in the program, unique identifiers are assigned to each set, and the instrumentation checks whether the current call target is a member of the expected call target set. If this check succeeds, then the original call is allowed to proceed, otherwise a failure handler is executed, which typically terminates the program.

In WebAssembly, the execution semantics implicitly guarantee the safety of (1) through usage of explicit function section indexes, and (3) through a protected call stack. Additionally, the type signature of indirect function calls is already checked at runtime, effectively implementing coarse-grained type-based control-flow integrity for (2). All of this is achieved without explicit runtime instrumentation in the module. However, as discussed previously, this protection does not prevent code reuse attacks with function-level granularity against indirect calls.

Clang/LLVM CFI

The Clang/LLVM compiler infrastructure includes a built-in implementation of fine-grained control flow integrity, which has been extended to support the WebAssembly target. It is available in Clang/LLVM 3.9+ with the new WebAssembly backend.

Enabling fine-grained control-flow integrity (by passing -fsanitize=cfi to emscripten) has a number of advantages over the default WebAssembly configuration. Not only does this better defend against code reuse attacks that leverage indirect function calls (2), but it also enhances the built-in function signature checks by operating at the C/C++ type level, which is semantically richer that the WebAssembly type level, which consists of only four value types. Currently, enabling this feature has a small performance cost for each indirect call, because an integer range check is used to verify that the target index is trusted, but this will be eliminated in the future by leveraging built-in support for multiple indirect tables with homogeneous type in WebAssembly.