bug#62039: Emacs crashes while parsing a long Emacs Lisp string

[Bruno Haible]

When parsing a particular long Emacs Lisp string, Emacs crashes.

How to reproduce:

$ emacs -Q -batch -f batch-byte-compile foo.el
Segmentation fault

Find attached the compressed file foo.el.

Emacs version: 27.1
Platform: x86_64-linux-gnu
$ ulimit -a | grep stack
stack size                  (kbytes, -s) 8192

According to the documentation
https://www.gnu.org/software/emacs/manual/html_node/emacs/Bug-Criteria.html
any segmentation fault is a bug.

I haven't analyzed the security impact of this bug, but it is quite possible
that emacs receives a string through the network, and even though the string
is not meant to be evaluated, simply parsing it causes a denial-of-service
to the emacs user.

The cause of the bug is that in emacs/src/lread.c the function read_escape()
is recursive, and no bound on the recursion depth is enforced.

foo.el.gz
Description: application/gzip