[ruby-core:103770] [Ruby master Feature#17837] Add support for Regexp timeouts

[mame@ruby-lang.org]

Issue #17837 has been updated by mame (Yusuke Endoh).


I've created a simple prototype of `Regexp.timeout=` by a polling approach.

Conclusion first. It brings about 5% overhead in micro benchmark, unfortunately.
I guess it is unlikely to be significant in a real application, but not good anyway.

---

The following is about my patch, just for the record.

https://github.com/ruby/ruby/compare/master...mame:regexp-timeout-prototype

Implementation approach:

1. When starting regexp matching, the current time is recorded by using `clock_gettime(CLOCK_MONOTONIC)`
2. At `CHECK_INTERRUPT_IN_MATCH_AT`, the elapsed time is calculated and an exception is raised if expired

Example:

```
Regexp.timeout = 1 # one second
/^(([a-z])+)+$/ =~ "abcdefghijklmnopqrstuvwxyz@" #=> regexp match timeout (RuntimeError)
```

Benchmark:

The following simple regexp matching becomes 4.8% slower.

```
10000000.times { /(abc)+/ =~ "abcabcabc" }

# The minimum time in 10 executions
# before: 1.962 s
# after:  2.056 s
```

The following complex regexp matching becomes 5.2% slower.

```
/^(([a-z])+)+$/ =~ "abcdefghijklmnopqrstuvwxyz@"

# The minimum time in 10 executions
# before: 2.237 s
# after:  2.353 s
```

----------------------------------------
Feature #17837: Add support for Regexp timeouts
https://bugs.ruby-lang.org/issues/17837#change-91882

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
----------------------------------------
### Background

ReDoS are a very common security issue. At Discourse we have seen a few through the years. https://owasp.org/www-community/attacks/Regular_expression_Denial_of_Service_-_ReDoS

In a nutshell there are 100s of ways this can happen in production apps, the key is for an attacker (or possibly innocent person) to supply either a problematic Regexp or a bad string to test it with.

```
/A(B|C+)+D/ =~ "A" + "C" * 100 + "X"
```

Having a problem Regexp somewhere in a large app is a universal constant, it will happen as long as you are using Regexps. 


Currently the only feasible way of supplying a consistent safeguard is by using `Thread.raise` and managing all execution. This kind of pattern requires usage of a third party implementation. There are possibly issues with jRuby and Truffle when taking approaches like this.

### Prior art

.NET provides a `MatchTimeout` property per: https://docs.microsoft.com/en-us/dotnet/api/system.text.regularexpressions.regex.matchtimeout?view=net-5.0

Java has nothing built in as far as I can tell: https://stackoverflow.com/questions/910740/cancelling-a-long-running-regex-match

Node has nothing built in as far as I can tell: https://stackoverflow.com/questions/38859506/cancel-regex-match-if-timeout


Golang and Rust uses RE2 which is not vulnerable to DoS by limiting features (available in Ruby RE2 gem)

```
irb(main):003:0> r = RE2::Regexp.new('A(B|C+)+D')
=> #<RE2::Regexp /A(B|C+)+D/>
irb(main):004:0> r.match("A" + "C" * 100 + "X")
=> nil
```

### Proposal

Implement `Regexp.timeout` which allow us to specify a global timeout for all Regexp operations in Ruby. 

Per Regexp would require massive application changes, almost all web apps would do just fine with a 1 second Regexp timeout.

If `timeout` is set to `nil` everything would work as it does today, when set to second a "monitor" thread would track running regexps and time them out according to the global value.

### Alternatives 

I recommend against a "per Regexp" API as this decision is at the application level. You want to apply it to all regular expressions in all the gems you are consuming.

I recommend against a move to RE2 at the moment as way too much would break 


### See also: 

https://people.cs.vt.edu/davisjam/downloads/publications/Davis-Dissertation-2020.pdf
https://levelup.gitconnected.com/the-regular-expression-denial-of-service-redos-cheat-sheet-a78d0ed7d865





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