whoops, don't discard runtime errors

boa_engine/src/context/mod.rs

@@ -97,6 +97,10 @@ pub struct Context {
     #[cfg(feature = "intl")]
     icu: icu::Icu,
 
+    /// Number of instructions remaining before a forced exit
+    #[cfg(feature = "fuzz")]
+    pub(crate) insns_remaining: usize,
+
     pub(crate) vm: Vm,
 
     pub(crate) promise_job_queue: VecDeque<JobCallback>,
@@ -580,6 +584,8 @@ pub struct ContextBuilder {
     interner: Option<Interner>,
     #[cfg(feature = "intl")]
     icu: Option<icu::Icu>,
+    #[cfg(feature = "fuzz")]
+    insns_remaining: usize,
 }
 
 impl ContextBuilder {
@@ -602,6 +608,15 @@ impl ContextBuilder {
         Ok(self)
     }
 
+    /// Specifies the number of instructions remaining to the [`Context`].
+    ///
+    /// This function is only available if the `fuzz` feature is enabled.
+    #[cfg(feature = "fuzz")]
+    pub fn insns_remaining(mut self, insns_remaining: usize) -> Self {
+        self.insns_remaining = insns_remaining;
+        self
+    }
+
     /// Creates a new [`ContextBuilder`] with a default empty [`Interner`]
     /// and a default [`BoaProvider`] if the `intl` feature is enabled.
     pub fn new() -> Self {
@@ -630,6 +645,8 @@ impl ContextBuilder {
                 icu::Icu::new(Box::new(icu_testdata::get_provider()))
                     .expect("Failed to initialize default icu data.")
             }),
+            #[cfg(feature = "fuzz")]
+            insns_remaining: self.insns_remaining,
             promise_job_queue: VecDeque::new(),
         };
 

boa_engine/src/error.rs

@@ -503,6 +503,17 @@ impl JsNativeError {
         Self::new(JsNativeErrorKind::Uri, Box::default(), None)
     }
 
+    /// Creates a new `JsNativeError` that indicates that the context hit its execution limit. This
+    /// is only used in a fuzzing context.
+    #[cfg(feature = "fuzz")]
+    pub fn no_instructions_remain() -> Self {
+        Self::new(
+            JsNativeErrorKind::NoInstructionsRemain,
+            Box::default(),
+            None,
+        )
+    }
+
     /// Sets the message of this error.
     ///
     /// # Examples
@@ -619,6 +630,12 @@ impl JsNativeError {
             }
             JsNativeErrorKind::Type => (constructors.type_error().prototype(), ErrorKind::Type),
             JsNativeErrorKind::Uri => (constructors.uri_error().prototype(), ErrorKind::Uri),
+            #[cfg(feature = "fuzz")]
+            JsNativeErrorKind::NoInstructionsRemain => {
+                // we can propagate out from try/catch since the catch block will also perform some
+                // operation
+                (constructors.error().prototype(), ErrorKind::Error)
+            }
         };
 
         let o = JsObject::from_proto_and_data(prototype, ObjectData::error(tag));
@@ -747,6 +764,10 @@ pub enum JsNativeErrorKind {
     /// [e_uri]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI
     /// [d_uri]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURI
     Uri,
+    /// Error thrown when no instructions remain. Only used in a fuzzing context; not a valid JS
+    /// error variant.
+    #[cfg(feature = "fuzz")]
+    NoInstructionsRemain,
 }
 
 impl std::fmt::Display for JsNativeErrorKind {
@@ -760,6 +781,8 @@ impl std::fmt::Display for JsNativeErrorKind {
             JsNativeErrorKind::Syntax => "SyntaxError",
             JsNativeErrorKind::Type => "TypeError",
             JsNativeErrorKind::Uri => "UriError",
+            #[cfg(feature = "fuzz")]
+            JsNativeErrorKind::NoInstructionsRemain => "NoInstructionsRemain",
         }
         .fmt(f)
     }

boa_engine/src/vm/mod.rs

@@ -5,7 +5,7 @@
 use crate::{
     builtins::async_generator::{AsyncGenerator, AsyncGeneratorState},
     vm::{call_frame::CatchAddresses, code_block::Readable},
-    Context, JsResult, JsValue,
+    Context, JsError, JsNativeError, JsResult, JsValue,
 };
 use boa_interner::ToInternedString;
 use boa_profiler::Profiler;
@@ -112,6 +112,13 @@ pub(crate) enum ReturnType {
 
 impl Context {
     fn execute_instruction(&mut self) -> JsResult<ShouldExit> {
+        #[cfg(feature = "fuzz")]
+        if self.insns_remaining == 0 {
+            return Err(JsError::from_native(JsNativeError::no_instructions_remain()));
+        } else {
+            self.insns_remaining -= 1;
+        }
+
         let opcode: Opcode = {
             let _timer = Profiler::global().start_event("Opcode retrieval", "vm");
             let opcode = self.vm.frame().code.code[self.vm.frame().pc]

fuzz/Cargo.toml

@@ -27,3 +27,9 @@ name = "parser-idempotency"
 path = "fuzz_targets/parser-idempotency.rs"
 test = false
 doc = false
+
+[[bin]]
+name = "vm-implied"
+path = "fuzz_targets/vm-implied.rs"
+test = false
+doc = false

fuzz/README.md

@@ -35,3 +35,16 @@ following:
      information, as the inputs parsed between the two should be the same.
 
 In this way, this fuzzer can identify correctness issues present in the parser.
+
+## VM Fuzzer
+
+The VM fuzzer, located in [vm-implied.rs](fuzz_targets/vm-implied.rs), identifies crash cases in the VM. It does so by
+generating an arbitrary AST, converting it to source code (to remove invalid inputs), then executing that source code.
+Because we are not comparing against any invariants other than "does it crash", this fuzzer will only discover faults
+which cause the VM to terminate unexpectedly, e.g. as a result of a panic. It will not discover logic errors present in
+the VM.
+
+To ensure that the VM does not attempt to execute an infinite loop, Boa is restricted to a finite number of instructions
+before the VM is terminated. If a program takes more than a second or so to execute, it likely indicates an issue in the
+VM (as we expect the fuzzer to execute only a certain amount of instructions, which should take significantly less
+time).

fuzz/fuzz_targets/common.rs

@@ -2,7 +2,7 @@ use boa_ast::{
     visitor::{VisitWith, VisitorMut},
     Expression, StatementList,
 };
-use boa_interner::{Interner, Sym};
+use boa_interner::{Interner, Sym, ToInternedString};
 use libfuzzer_sys::arbitrary;
 use libfuzzer_sys::arbitrary::{Arbitrary, Unstructured};
 use std::fmt::{Debug, Formatter};
@@ -72,3 +72,25 @@ impl Debug for FuzzData {
             .finish_non_exhaustive()
     }
 }
+
+pub struct FuzzSource {
+    pub interner: Interner,
+    pub source: String,
+}
+
+impl<'a> Arbitrary<'a> for FuzzSource {
+    fn arbitrary(u: &mut Unstructured<'a>) -> arbitrary::Result<Self> {
+        let data = FuzzData::arbitrary(u)?;
+        let source = data.ast.to_interned_string(&data.interner);
+        Ok(Self {
+            interner: data.interner,
+            source,
+        })
+    }
+}
+
+impl Debug for FuzzSource {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.write_fmt(format_args!("Fuzzed source:\n{}", self.source))
+    }
+}

fuzz/fuzz_targets/vm-implied.rs

@@ -0,0 +1,19 @@
+#![no_main]
+
+mod common;
+
+use crate::common::FuzzSource;
+use boa_engine::{Context, JsResult, JsValue};
+use libfuzzer_sys::fuzz_target;
+
+fn do_fuzz(original: FuzzSource) -> JsResult<JsValue> {
+    let mut ctx = Context::builder()
+        .interner(original.interner)
+        .insns_remaining(1 << 16)
+        .build();
+    ctx.eval(&original.source)
+}
+
+fuzz_target!(|original: FuzzSource| {
+    let _ = do_fuzz(original);
+});