[ruby-core:105168] [Ruby master Bug#18138] Array#slice! invalid memory access

Issue #18138 has been updated by mdalessio (Mike Dalessio).


A colleague asked if this bug should have a CVE number, given that any application that might take offsets as untrusted input could be tricked into:

- accessing unrelated memory - CWE-125 (https://cwe.mitre.org/data/definitions/125.html)
- crashing (Denial of Service) - CWE-248 (https://cwe.mitre.org/data/definitions/248.html)

I'm not a security professional, but it does seem to me as though a CVE should be discussed.


----------------------------------------
Bug #18138: Array#slice! invalid memory access
https://bugs.ruby-lang.org/issues/18138#change-93573

* Author: mdalessio (Mike Dalessio)
* Status: Closed
* Priority: Normal
* ruby -v: ruby 3.1.0dev (2021-08-28T14:40:37Z master 808ce96494) [x86_64-linux]
* Backport: 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONE
----------------------------------------
As of 4f24255, the array.c functions rb_ary_slice_bang / ary_slice_bang_by_rb_ary_splice allow a length to be passed to rb_ary_new4 that is too long and which leads to an invalid memory access.

This bug is present in Ruby v3_0_0, v3_0_1, and v3_0_2.


## Reproduction

This ruby snippet will reproduce valgrind memory warnings:

``` ruby
(1..5000).to_a.slice!(-2, 5000)
```

The valgrind memory warnings on `master` look like:

``` text
==228628== Invalid read of size 8
==228628==    at 0x48428C0: memmove (vg_replace_strmem.c:1271)
==228628==    by 0x356542: ary_memcpy (array.c:316)
==228628==    by 0x356542: rb_ary_tmp_new_from_values (array.c:785)
==228628==    by 0x356542: rb_ary_new_from_values (array.c:795)
==228628==    by 0x356542: ary_slice_bang_by_rb_ary_splice (array.c:4106)
==228628==    by 0x35E1DB: rb_ary_slice_bang (array.c:4186)
```

## Fix

The fix I'm suggesting is in pull request https://github.com/ruby/ruby/pull/4787

Saving you a click:


``` text
diff --git a/array.c b/array.c
index bd323cd..edac216 100644
--- a/array.c
+++ b/array.c
@@ -4096,7 +4096,7 @@ ary_slice_bang_by_rb_ary_splice(VALUE ary, long pos, long len)
     else if (orig_len < pos) {
         return Qnil;
     }
-    else if (orig_len < pos + len) {
+    if (orig_len < pos + len) {
         len = orig_len - pos;
     }
     if (len == 0) {
```




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