Limit expression/statement nesting depths.

README.md

@@ -24,7 +24,7 @@ Rhai's current features set:
   one single source file, all with names starting with `"unsafe_"`).
 * Re-entrant scripting [`Engine`] can be made `Send + Sync` (via the [`sync`] feature).
 * Sand-boxed - the scripting [`Engine`], if declared immutable, cannot mutate the containing environment without explicit permission.
-* Rugged (protection against [stack-overflow](#maximum-stack-depth) and [runaway scripts](#maximum-number-of-operations) etc.).
+* Rugged (protection against [stack-overflow](#maximum-call-stack-depth) and [runaway scripts](#maximum-number-of-operations) etc.).
 * Track script evaluation [progress](#tracking-progress) and manually terminate a script run.
 * [`no-std`](#optional-features) support.
 * [Function overloading](#function-overloading).
@@ -1066,12 +1066,13 @@ fn main() -> Result<(), Box<EvalAltResult>>
 Engine configuration options
 ---------------------------
 
-| Method                   | Description                                                                              |
-| ------------------------ | ---------------------------------------------------------------------------------------- |
-| `set_optimization_level` | Set the amount of script _optimizations_ performed. See [`script optimization`].         |
-| `set_max_call_levels`    | Set the maximum number of function call levels (default 50) to avoid infinite recursion. |
-
-[`script optimization`]: #script-optimization
+| Method                   | Description                                                                                                                                         |
+| ------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `set_optimization_level` | Set the amount of script _optimizations_ performed. See [script optimization].                                                                      |
+| `set_max_expr_depths`    | Set the maximum nesting levels of an expression/statement. See [maximum statement depth](#maximum-statement-depth).                                 |
+| `set_max_call_levels`    | Set the maximum number of function call levels (default 50) to avoid infinite recursion. See [maximum call stack depth](#maximum-call-stack-depth). |
+| `set_max_operations`     | Set the maximum number of _operations_ that a script is allowed to consume. See [maximum number of operations](#maximum-number-of-operations).      |
+| `set_max_modules`        | Set the maximum number of [modules] that a script is allowed to load. See [maximum number of modules](#maximum-number-of-modules).                  |
 
 -------
 
@@ -2267,9 +2268,12 @@ so that it does not consume more resources that it is allowed to.
 The most important resources to watch out for are:
 
 * **Memory**: A malignant script may continuously grow an [array] or [object map] until all memory is consumed.
+  It may also create a large [array] or [objecct map] literal that exhausts all memory during parsing.
 * **CPU**: A malignant script may run an infinite tight loop that consumes all CPU cycles.
 * **Time**: A malignant script may run indefinitely, thereby blocking the calling system which is waiting for a result.
 * **Stack**: A malignant script may attempt an infinite recursive call that exhausts the call stack.
+  Alternatively, it may create a degenerated deep expression with so many levels that the parser exhausts the call stack
+  when parsing the expression; or even deeply-nested statement blocks, if nested deep enough.
 * **Overflows**: A malignant script may deliberately cause numeric over-flows and/or under-flows, divide by zero, and/or
   create bad floating-point representations, in order to crash the system.
 * **Files**: A malignant script may continuously [`import`] an external module within an infinite loop,
@@ -2341,10 +2345,15 @@ engine.set_max_modules(5);                  // allow loading only up to 5 module
 engine.set_max_modules(0);                  // allow unlimited modules
 ```
 
-### Maximum stack depth
+### Maximum call stack depth
 
-Rhai by default limits function calls to a maximum depth of 256 levels (28 levels in debug build).
+Rhai by default limits function calls to a maximum depth of 128 levels (8 levels in debug build).
 This limit may be changed via the `Engine::set_max_call_levels` method.
+
+When setting this limit, care must be also taken to the evaluation depth of each _statement_
+within the function. It is entirely possible for a malignant script to embed an recursive call deep
+inside a nested expression or statement block (see [maximum statement depth](#maximum-statement-depth)).
+
 The limit can be disabled via the [`unchecked`] feature for higher performance
 (but higher risks as well).
 
@@ -2358,12 +2367,57 @@ engine.set_max_call_levels(0);              // allow no function calls at all (m
 
 A script exceeding the maximum call stack depth will terminate with an error result.
 
+### Maximum statement depth
+
+Rhai by default limits statements and expressions nesting to a maximum depth of 128
+(which should be plenty) when they are at _global_ level, but only a depth of 32
+when they are within function bodies.  For debug builds, these limits are set further
+downwards to 32 and 16 respectively.
+
+That is because it is possible to overflow the [`Engine`]'s stack when it tries to
+recursively parse an extremely deeply-nested code stream.
+
+```rust
+// The following, if long enough, can easily cause stack overflow during parsing.
+let a = (1+(1+(1+(1+(1+(1+(1+(1+(1+(1+(...)+1)))))))))));
+```
+
+This limit may be changed via the `Engine::set_max_expr_depths` method.  There are two limits to set,
+one for the maximum depth at global level, and the other for function bodies.
+
+```rust
+let mut engine = Engine::new();
+
+engine.set_max_expr_depths(50, 5);          // allow nesting up to 50 layers of expressions/statements
+                                            // at global level, but only 5 inside functions
+```
+
+Beware that there may be multiple layers for a simple language construct, even though it may correspond
+to only one AST node. That is because the Rhai _parser_ internally runs a recursive chain of function calls
+and it is important that a malignant script does not panic the parser in the first place.
+
+Functions are placed under stricter limits because of the multiplicative effect of recursion.
+A script can effectively call itself while deep inside an expression chain within the function body,
+thereby overflowing the stack even when the level of recursion is within limit.
+
+Make sure that `C x ( 5 + F ) + S` layered calls do not cause a stack overflow, where:
+
+* `C` = maximum call stack depth,
+* `F` = maximum statement depth for functions,
+* `S` = maximum statement depth at global level.
+
+A script exceeding the maximum nesting depths will terminate with a parsing error.
+The malignant `AST` will not be able to get past parsing in the first place.
+
+The limits can be disabled via the [`unchecked`] feature for higher performance
+(but higher risks as well).
+
 ### Checked arithmetic
 
-All arithmetic calculations in Rhai are _checked_, meaning that the script terminates with an error whenever
-it detects a numeric over-flow/under-flow condition or an invalid floating-point operation, instead of
-crashing the entire system.  This checking can be turned off via the [`unchecked`] feature for higher performance
-(but higher risks as well).
+By default, all arithmetic calculations in Rhai are _checked_, meaning that the script terminates
+with an error whenever it detects a numeric over-flow/under-flow condition or an invalid
+floating-point operation, instead of crashing the entire system.  This checking can be turned off
+via the [`unchecked`] feature for higher performance (but higher risks as well).
 
 ### Blocking access to external data
 
@@ -2383,6 +2437,8 @@ let engine = engine;                        // shadow the variable so that 'engi
 Script optimization
 ===================
 
+[script optimization]: #script-optimization
+
 Rhai includes an _optimizer_ that tries to optimize a script after parsing.
 This can reduce resource utilization and increase execution speed.
 Script optimization can be turned off via the [`no_optimize`] feature.

RELEASES.md

@@ -1,6 +1,15 @@
 Rhai Release Notes
 ==================
 
+Version 0.15.0
+==============
+
+New features
+------------
+
+* Set limits on maximum level of nesting expressions and statements to avoid panics during parsing.
+
+
 Version 0.14.1
 ==============
 

src/api.rs

@@ -444,7 +444,14 @@ impl Engine {
         optimization_level: OptimizationLevel,
     ) -> Result<AST, Box<ParseError>> {
         let stream = lex(scripts);
-        parse(&mut stream.peekable(), self, scope, optimization_level)
+
+        parse(
+            &mut stream.peekable(),
+            self,
+            scope,
+            optimization_level,
+            (self.max_expr_depth, self.max_function_expr_depth),
+        )
     }
 
     /// Read the contents of a file into a string.
@@ -571,6 +578,7 @@ impl Engine {
             self,
             &scope,
             OptimizationLevel::None,
+            self.max_expr_depth,
         )?;
 
         // Handle null - map to ()
@@ -654,7 +662,13 @@ impl Engine {
 
         {
             let mut peekable = stream.peekable();
-            parse_global_expr(&mut peekable, self, scope, self.optimization_level)
+            parse_global_expr(
+                &mut peekable,
+                self,
+                scope,
+                self.optimization_level,
+                self.max_expr_depth,
+            )
         }
     }
 
@@ -805,8 +819,14 @@ impl Engine {
     ) -> Result<T, Box<EvalAltResult>> {
         let scripts = [script];
         let stream = lex(&scripts);
-        // Since the AST will be thrown away afterwards, don't bother to optimize it
-        let ast = parse_global_expr(&mut stream.peekable(), self, scope, OptimizationLevel::None)?;
+
+        let ast = parse_global_expr(
+            &mut stream.peekable(),
+            self,
+            scope,
+            self.optimization_level,
+            self.max_expr_depth,
+        )?;
         self.eval_ast_with_scope(scope, &ast)
     }
 
@@ -931,8 +951,13 @@ impl Engine {
         let scripts = [script];
         let stream = lex(&scripts);
 
-        // Since the AST will be thrown away afterwards, don't bother to optimize it
-        let ast = parse(&mut stream.peekable(), self, scope, OptimizationLevel::None)?;
+        let ast = parse(
+            &mut stream.peekable(),
+            self,
+            scope,
+            self.optimization_level,
+            (self.max_expr_depth, self.max_function_expr_depth),
+        )?;
         self.consume_ast_with_scope(scope, &ast)
     }
 

src/engine.rs

@@ -49,14 +49,30 @@ pub type Map = HashMap<String, Dynamic>;
 
 #[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
-pub const MAX_CALL_STACK_DEPTH: usize = 28;
+pub const MAX_CALL_STACK_DEPTH: usize = 8;
+#[cfg(not(feature = "unchecked"))]
+#[cfg(debug_assertions)]
+pub const MAX_EXPR_DEPTH: usize = 32;
+#[cfg(not(feature = "unchecked"))]
+#[cfg(debug_assertions)]
+pub const MAX_FUNCTION_EXPR_DEPTH: usize = 16;
 
 #[cfg(not(feature = "unchecked"))]
 #[cfg(not(debug_assertions))]
-pub const MAX_CALL_STACK_DEPTH: usize = 256;
+pub const MAX_CALL_STACK_DEPTH: usize = 128;
+#[cfg(not(feature = "unchecked"))]
+#[cfg(not(debug_assertions))]
+pub const MAX_EXPR_DEPTH: usize = 128;
+#[cfg(not(feature = "unchecked"))]
+#[cfg(not(debug_assertions))]
+pub const MAX_FUNCTION_EXPR_DEPTH: usize = 32;
 
 #[cfg(feature = "unchecked")]
 pub const MAX_CALL_STACK_DEPTH: usize = usize::MAX;
+#[cfg(feature = "unchecked")]
+pub const MAX_EXPR_DEPTH: usize = usize::MAX;
+#[cfg(feature = "unchecked")]
+pub const MAX_FUNCTION_EXPR_DEPTH: usize = usize::MAX;
 
 pub const KEYWORD_PRINT: &str = "print";
 pub const KEYWORD_DEBUG: &str = "debug";
@@ -338,8 +354,12 @@ pub struct Engine {
     pub(crate) optimization_level: OptimizationLevel,
     /// Maximum levels of call-stack to prevent infinite recursion.
     ///
-    /// Defaults to 28 for debug builds and 256 for non-debug builds.
+    /// Defaults to 8 for debug builds and 128 for non-debug builds.
     pub(crate) max_call_stack_depth: usize,
+    /// Maximum depth of statements/expressions at global level.
+    pub(crate) max_expr_depth: usize,
+    /// Maximum depth of statements/expressions in functions.
+    pub(crate) max_function_expr_depth: usize,
     /// Maximum number of operations allowed to run.
     pub(crate) max_operations: Option<NonZeroU64>,
     /// Maximum number of modules allowed to load.
@@ -382,6 +402,8 @@ impl Default for Engine {
             optimization_level: OptimizationLevel::Full,
 
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
+            max_expr_depth: MAX_EXPR_DEPTH,
+            max_function_expr_depth: MAX_FUNCTION_EXPR_DEPTH,
             max_operations: None,
             max_modules: None,
         };
@@ -523,6 +545,8 @@ impl Engine {
             optimization_level: OptimizationLevel::Full,
 
             max_call_stack_depth: MAX_CALL_STACK_DEPTH,
+            max_expr_depth: MAX_EXPR_DEPTH,
+            max_function_expr_depth: MAX_FUNCTION_EXPR_DEPTH,
             max_operations: None,
             max_modules: None,
         }
@@ -574,6 +598,13 @@ impl Engine {
         self.max_modules = NonZeroU64::new(modules);
     }
 
+    /// Set the depth limits for expressions/statements.
+    #[cfg(not(feature = "unchecked"))]
+    pub fn set_max_expr_depths(&mut self, max_expr_depth: usize, max_function_expr_depth: usize) {
+        self.max_expr_depth = max_expr_depth;
+        self.max_function_expr_depth = max_function_expr_depth;
+    }
+
     /// Set the module resolution service used by the `Engine`.
     ///
     /// Not available under the `no_module` feature.

src/error.rs

@@ -110,6 +110,10 @@ pub enum ParseErrorType {
     AssignmentToCopy,
     /// Assignment to an a constant variable.
     AssignmentToConstant(String),
+    /// Expression exceeding the maximum levels of complexity.
+    ///
+    /// Never appears under the `unchecked` feature.
+    ExprTooDeep,
     /// Break statement not inside a loop.
     LoopBreak,
 }
@@ -158,7 +162,8 @@ impl ParseError {
             ParseErrorType::DuplicatedExport(_) => "Duplicated variable/function in export statement",
             ParseErrorType::WrongExport => "Export statement can only appear at global level",
             ParseErrorType::AssignmentToCopy => "Only a copy of the value is change with this assignment",
-            ParseErrorType::AssignmentToConstant(_) => "Cannot assign to a constant value.",
+            ParseErrorType::AssignmentToConstant(_) => "Cannot assign to a constant value",
+            ParseErrorType::ExprTooDeep => "Expression exceeds maximum complexity",
             ParseErrorType::LoopBreak => "Break statement should only be used inside a loop"
         }
     }