CVE-2023-48706 (C, published 2023-11-22T22:15Z)
Description: value: Vim is a UNIX editor that, prior to version 9.0.2121, has a heap-use-after-free vulnerability. When executing a `:s` command for the very first time and using a sub-replace-special atom inside the substitution part, it is possible that the recursive `:s` call causes free-ing of memory which may later then be accessed by the initial `:s` command. The user must intentionally execute the payload and the whole process is a bit tricky to do since it seems to work only reliably for the very first :s command. It may also cause a crash of Vim. Version 9.0.2121 contains a fix for this issue.
Severity: nan (CVSS v3 base score: 4.7)
Weakness: CWE-416 \u2014 Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Repository: https://github.com/vim/vim
Files changed:
src/ex_cmds.c
src/testdir/test_substitute.vim
src/version.c
Commit message: patch 9.0.2121: [security]: use-after-free in ex_substitute

Problem:  [security]: use-after-free in ex_substitute
Solution: always allocate memory

closes: #13552

A recursive :substitute command could cause a heap-use-after free in Vim
(CVE-2023-48706).

The whole reproducible test is a bit tricky, I can only reproduce this
reliably when no previous substitution command has been used yet
(which is the reason, the test needs to run as first one in the
test_substitute.vim file) and as a combination of the `:~` command
together with a :s command that contains the special substitution atom `~\=`
which will make use of a sub-replace special atom and calls a vim script
function.

There was a comment in the existing :s code, that already makes the
`sub` variable allocate memory so that a recursive :s call won't be able
to cause any issues here, so this was known as a potential problem
already.  But for the current test-case that one does not work, because
the substitution does not start with `\=` but with `~\=` (and since
there does not yet exist a previous substitution atom, Vim will simply
increment the `sub` pointer (which then was not allocated dynamically)
and later one happily use a sub-replace special expression (which could
then free the `sub` var).

The following commit fixes this, by making the sub var always using
allocated memory, which also means we need to free the pointer whenever
we leave the function. Since sub is now always an allocated variable,
we also do no longer need the sub_copy variable anymore, since this one
was used to indicated when sub pointed to allocated memory (and had
therefore to be freed on exit) and when not.

Github Security Advisory:
https://github.com/vim/vim/security/advisories/GHSA-c8qm-x72m-q53q

Signed-off-by: Christian Brabandt <cb@256bit.org>


Vulnerable code:
    char_u	*sub_copy = NULL;
	sub = cmd;	    // remember the start of the substitution
		sub = old_sub;
	sub = old_sub;
	sub = vim_strsave(sub);
	if (sub == NULL)
	sub_copy = sub;
	char_u *newsub = regtilde(sub, magic_isset());
	if (newsub != sub)
	    // newsub was allocated, free it later.
	    sub_copy = newsub;
	    sub = newsub;
    vim_free(sub_copy);

  put ="abcdef"

Fixed code:
    char_u	*p;
	p = cmd;	    // remember the start of the substitution
	sub = vim_strsave(p);
	if (sub == NULL)
	    // out of memory
	    return;
		    vim_free(sub);
		vim_free(sub);
		sub = vim_strsave(old_sub);
		if (sub == NULL)
		    // out of memory
		    return;
		if (old_sub == NULL)
		    // out of memory
		    return;
	sub = vim_strsave(old_sub);
	{
	    vim_free(sub);
	}
	vim_free(sub);
	    vim_free(sub);
	    vim_free(sub);
	    vim_free(sub);
    {
	vim_free(sub);
    }
	vim_free(sub);
	vim_free(sub);
	p = vim_strsave(sub);
	vim_free(sub);
	if (p == NULL)
	sub = p;
	p = regtilde(sub, magic_isset());
	if (p != sub)
	    vim_free(sub);
	    sub = p;
    vim_free(sub);
" NOTE: This needs to be the first test to be
"       run in the file, since it depends on
"       that the previous substitution atom
"       was not yet set.
"
" recursive call of :s and sub-replace special
" (did cause heap-use-after free in < v9.0.2121)
func Test_aaaa_substitute_expr_recursive_special()
  func R()
    " FIXME: leaving out the 'n' flag leaks memory, why?
    %s/./\='.'/gn
  endfunc
  new Xfoobar_UAF
  put ='abcdef'
  let bufnr = bufnr('%')
  try
    silent! :s/./~\=R()/0
    "call assert_fails(':s/./~\=R()/0', 'E939:')
    let @/='.'
    ~g
  catch /^Vim\%((\a\+)\)\=:E565:/
  endtry
  delfunc R
  exe bufnr .. "bw!"
endfunc

  put ='abcdef'
" recursive call of :s using test-replace special
func Test_substitute_expr_recursive()
  func Q()
    %s/./\='foobar'/gn
    return "foobar"
  endfunc
  func R()
    %s/./\=Q()/g
  endfunc
  new Xfoobar_UAF
  let bufnr = bufnr('%')
  put ='abcdef'
  silent! s/./\=R()/g
  call assert_fails(':%s/./\=R()/g', 'E565:')
  delfunc R
  delfunc Q
  exe bufnr .. "bw!"
endfunc

/**/
    2121,