From: XrXr@...
Date: 2019-09-23T00:25:27+00:00
Subject: [ruby-core:95040] [Ruby master Bug#16151] [PATCH] Fix a class of fstring related use-after-free

Issue #16151 has been updated by alanwu (Alan Wu).


I ran some quick tests on rejecting frozen non-bare strings from the buffer
deduplication code path. On Discourse, about 18% of calls to rb_fstring are
refused with the change in https://bugs.ruby-lang.org/issues/16151#note-5.
So I don't think that's a viable approach.

I've put together a PR with nobu's idea, using a new flag to make sure to
not defeat the optimization in #13085. I also took the liberty to rename
STR_IS_SHARED_M since it's a little confusing to have this many things talk
about sharing.

https://github.com/ruby/ruby/pull/2480


----------------------------------------
Bug #16151: [PATCH] Fix a class of fstring related use-after-free
https://bugs.ruby-lang.org/issues/16151#change-81671

* Author: alanwu (Alan Wu)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-09-07T18:26:35Z master e9bc8b35c6) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
Pull request: https://github.com/ruby/ruby/pull/2435

## The bug

Run the following against master(e9bc8b3) to observe use-after-free:

```ruby
-('a' * 30).force_encoding(Encoding::ASCII)
a = ('a' * 30).force_encoding(Encoding::ASCII).taint

t = Thread.new{}
t.name = a
eval('', binding, t.name)
p a
```

```ruby
-('a' * 30).force_encoding(Encoding::ASCII)
a = ('a' * 30).force_encoding(Encoding::ASCII).taint

require 'ripper'
ripper = Ripper.new("", a)
eval('', binding, ripper.filename)
p a
```

There may be other cases in the standard library or in the wild.

## Background

When a string has both `STR_NOEMBED` and `STR_SHARED` set, it relies on a
different string for its buffer. I will refer to strings that are depended upon
as "shared roots". Shared roots are frozen and have the `STR_SHARED` unset.
This is a bit unintuitive to me. A name for `STR_SHARED` that makes more sense
to me would be `STR_BUFFER_ELSEWHERE`.

## What went wrong

It is not safe to free the buffer of a shared root while it has dependants. The
root and its dependants use the same buffer. As such, it is only safe to free
the shared buffer when all users are unreachable on the heap.

## The Fix

`rb_fstring` has a code path that frees and replaces the buffer of its input.
Using this code path on the shared root of dependant strings sets up
use-after-free. This patch removes the problematic code path as no tests
require said buffer replacement functionality. Additionally, there has been
three other issues that steam from this particular code path. See #15926,
#15916 and #16136

---

I used @mame's commit in #16136 as the starting point for this investigation.
Thank you!



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