From: sam.saffron@...
Date: 2021-05-11T07:33:58+00:00
Subject: [ruby-core:103780] [Ruby master Feature#17837] Add support for	Regexp timeouts

Issue #17837 has been updated by sam.saffron (Sam Saffron).


I tested with: 

```
diff --git a/thread.c b/thread.c
index 47e43ecb63..811b6e88a8 100644
--- a/thread.c
+++ b/thread.c
@@ -1573,25 +1573,29 @@ rb_thread_reg_match_time_limit_get()
 void
 rb_thread_reg_match_start(void)
 {
-    rb_thread_t *th = GET_THREAD();
     if (reg_match_time_limit) {
-        th->reg_match_end_time = rb_hrtime_add(reg_match_time_limit, rb_hrtime_now());
-    }
-    else {
-        th->reg_match_end_time = 0;
+       rb_thread_t *th = GET_THREAD();
+       if (reg_match_time_limit) {
+           th->reg_match_end_time = rb_hrtime_add(reg_match_time_limit, rb_hrtime_now());
+       }
+       else {
+           th->reg_match_end_time = 0;
+       }
     }
 }
 
 void
 rb_thread_reg_check_ints(void)
 {
-    rb_thread_t *th = GET_THREAD();
+    if (reg_match_time_limit) {
+       rb_thread_t *th = GET_THREAD();
 
-    if (th->reg_match_end_time && th->reg_match_end_time < rb_hrtime_now()) {
-        VALUE argv[2];
-        argv[0] = rb_eRuntimeError;
-        argv[1] = rb_str_new2("regexp match timeout");
-        rb_threadptr_raise(th, 2, argv);
+       if (th->reg_match_end_time && th->reg_match_end_time < rb_hrtime_now()) {
+           VALUE argv[2];
+           argv[0] = rb_eRuntimeError;
+           argv[1] = rb_str_new2("regexp match timeout");
+           rb_threadptr_raise(th, 2, argv);
+       }
     }
 
     rb_thread_check_ints();
```

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

Before (min over 10 runs): 1.590 after 1.610 ~ 1.2% slower


I can't figure out though how to squeeze back the perf on the big regex.

----------------------------------------
Feature #17837: Add support for Regexp timeouts
https://bugs.ruby-lang.org/issues/17837#change-91893

* 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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