vrl::compile: attempt to subtract with overflow

[stasos24]

Hello, i've found a secureity bug during fuzzing vector.

Version: v0.23.0

Steps to reproduce:
Download a repo with fuzz harness: https://github.com/stasos24/vector/tree/master/lib/vrl/core/fuzzing

cargo build --bin covbuild
target/debug/covbuild < crash.file

Link to crash.file:
crash_file

Rust backtrace:

thread 'main' panicked at 'attempt to subtract with overflow', /opt/fuzz/vector/lib/vrl/compiler/src/expression/assignment.rs:250:37
stack backtrace:
   0: rust_begin_unwind
             at /rustc/38b72154ded23847cd08a796d0c6708b5efac265/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/38b72154ded23847cd08a796d0c6708b5efac265/library/core/src/panicking.rs:142:14
   2: core::panicking::panic
             at /rustc/38b72154ded23847cd08a796d0c6708b5efac265/library/core/src/panicking.rs:48:5
   3: vrl_compiler::expression::assignment::verify_overwriteable
             at /opt/fuzz/vector/lib/vrl/compiler/src/expression/assignment.rs:250:37
   4: vrl_compiler::expression::assignment::Assignment::new
             at /opt/fuzz/vector/lib/vrl/compiler/src/expression/assignment.rs:64:17
   5: vrl_compiler::compiler::Compiler::compile_assignment
             at /opt/fuzz/vector/lib/vrl/compiler/src/compiler.rs:551:26
   6: vrl_compiler::compiler::Compiler::compile_expr
             at /opt/fuzz/vector/lib/vrl/compiler/src/compiler.rs:118:33
   7: vrl_compiler::compiler::Compiler::compile_root_exprs
             at /opt/fuzz/vector/lib/vrl/compiler/src/compiler.rs:231:41
   8: vrl_compiler::compiler::Compiler::compile
             at /opt/fuzz/vector/lib/vrl/compiler/src/compiler.rs:61:27
   9: vrl::compile_with_state
             at /opt/fuzz/vector/lib/vrl/vrl/src/lib.rs:45:5
  10: vrl::compile_with_external
             at /opt/fuzz/vector/lib/vrl/vrl/src/lib.rs:34:5
  11: vrl::compile
             at /opt/fuzz/vector/lib/vrl/vrl/src/lib.rs:26:5
  12: covbuild::main
             at ./src/cov.rs:14:3
  13: core::ops::function::FnOnce::call_once
             at /rustc/38b72154ded23847cd08a796d0c6708b5efac265/library/core/src/ops/function.rs:248:5

[fuchsnj]

This is likely a duplicate of vectordotdev/vector#13461

[stasos24]

Also found same error at:
thread 'main' panicked at 'attempt to subtract with overflow', /opt/fuzz/vector/lib/vrl/compiler/src/expression/assignment.rs:259:37
Input file:
crash_file
And at:
thread 'main' panicked at 'attempt to subtract with overflow', /opt/fuzz/vector/lib/vrl/compiler/src/expression/assignment.rs:253:62
Input file:
crash_file

[fuchsnj]

The panics here have been prevented by vectordotdev/vector#14114 and #207
There is still a known bug related to this behavior tracked here but it can no longer panic.