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


That general approach sounds good to me, but please let me mention some caveats.

```diff
@@ -1308,8 +1310,6 @@ str_new_frozen(VALUE klass, VALUE orig)
 	    RBASIC(str)->flags |= RBASIC(orig)->flags & STR_NOFREE;
 	    RBASIC(orig)->flags &= ~STR_NOFREE;
 	    STR_SET_SHARED(orig, str);
-	    if (klass == 0)
-		FL_UNSET_RAW(str, STR_IS_SHARED_M);
 	}
     }
 
```

These two lines are important for #13085, where `STR_IS_SHARED_M` was first
introduced. The following program generates a lot more garbage when I comment
out these two lines:

```ruby
# cat.rb
buf = ''.b
File.open(ARGV.first) do |f|
  while f.read(16384, buf)
    $stdout.write(buf)
  end
end

$stderr.puts(File.read("/proc/#{Process.pid}/status"))
```

Going with this approach while keeping the optimization introduced in #13085
might require a new flag.

Sidenote, marking roots with a flag is a heuristic, since a string stops being
a root once all the dependants are unreachable. We already have a more coarse
heuristic: if a string is frozen, we know that it could be a shared root.
Maybe that can be a viable alternative approach.

```diff
diff --git a/string.c b/string.c
index 05ce0ed8d6..86db4891fb 100644
--- a/string.c
+++ b/string.c
@@ -327,6 +327,8 @@ rb_fstring(VALUE str)
     fstr = register_fstring(str);
 
     if (!bare) {
+        /* A frozen string might be a shared root */
+        if (FL_TEST_RAW(str, FL_FREEZE)) return str;
        str_replace_shared_without_enc(str, fstr);
        OBJ_FREEZE_RAW(str);
        return str;
```

Though, I think if we can get away with removing the buffer replacement code path, we should.


----------------------------------------
Bug #16151: [PATCH] Fix a class of fstring related use-after-free
https://bugs.ruby-lang.org/issues/16151#change-81471

* 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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