Packer#write_extension: interior pointer of a coerced temporary held across rb_raise (use-after-move) #401

Summary

Packer_write_extension takes a char * into a String that rb_String has just created, and
passes it to rb_raise, which allocates while it formats. If a GC compaction runs during that
allocation the temporary can move, and %s then reads a vacated slot. This is a latent
memory-safety defect on the error path. It is present at HEAD (and in 1.8.4).

Location

ext/msgpack/packer_class.c (line numbers from 1.8.4, identical in 1.8.3):

static VALUE Packer_write_extension(VALUE self, VALUE obj)
{
    ...
    VALUE rb_ext_type = RSTRUCT_GET(obj, 0);
    if(!RB_TYPE_P(rb_ext_type, T_FIXNUM)) {
        rb_raise(rb_eRangeError, "integer %s too big to convert to `signed char'",
                 RSTRING_PTR(rb_String(rb_ext_type)));   /* :238 */
    }

When the type slot is not a Fixnum, rb_String(rb_ext_type) coerces it to a fresh String that
no argument slot holds (argv holds the ExtensionValue struct, not the coerced String).
RSTRING_PTR takes an interior pointer into that fresh String, and rb_raise → rb_vsprintf
allocates the message buffer before it reads %s. A compaction in that window relocates the fresh
String; the read lands in the vacated, zero-filled slot.

Reproduction

Reproduces on arm64-darwin, ruby 3.4.7/3.4.10/4.0.6: an ExtensionValue whose type slot is a
non-Fixnum object with an embedded (movable) to_str, packed under GC.stress + GC.auto_compact,
raises RangeError whose %s field is empty — "integer too big to convert to \signed
char'"— 200/200, versus the intact marker on the same run with the type slot already a String (0/200) or with GC off (0/200). Natural rate, noGC.stress`: ~35 corruptions in 300,000 packs.

It is codegen-dependent: on aarch64-linux with Debian gcc 12 the temporary stays in a
conservatively-scanned register across rb_raise and does not move (0/300 while a same-size witness
relocated 300/300), so this build is safe only by accident. A compiler, flag or arch change unpins
it.

Fix

Root the coerced value in a local before raising, so it is a named VALUE on the stack for the
duration of the rb_raise:

VALUE s = rb_String(rb_ext_type);
rb_raise(rb_eRangeError, "integer %"PRIsVALUE" too big to convert to `signed char'", s);

Using %"PRIsVALUE" with the VALUE (rather than %s with RSTRING_PTR) also removes the raw
pointer entirely.

Environment: msgpack 1.8.3 and 1.8.4, ruby 3.4.x, arm64-darwin (fires) and aarch64-linux (latent).