patch 9.0.2121: [security]: use-after-free in ex_substitute Commit: https://github.com/vim/vim/commit/26c11c56888d01e298cd8044caf860f3c26f57bb Author: Christian Brabandt <cb@256bit.org> Date: Wed Nov 22 21:26:41 2023 +0100 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>

--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -4,6 +4,32 @@ source shared.vim
 source check.vim
 source screendump.vim
 
+" 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
+
 func Test_multiline_subst()
   enew!
   call append(0, ["1 aa",
@@ -147,7 +173,6 @@ func Test_substitute_repeat()
   call feedkeys("Qsc\<CR>y", 'tx')
   bwipe!
 endfunc
-
 " Test %s/\n// which is implemented as a special case to use a
 " more efficient join rather than doing a regular substitution.
 func Test_substitute_join()
@@ -1447,11 +1472,30 @@ func Test_substitute_expr_switch_win()
   endfunc
   new Xfoobar
   let bufnr = bufnr('%')
-  put ="abcdef"
+  put ='abcdef'
   silent! s/\%')/\=R()
   call assert_fails(':%s/./\=R()/g', 'E565:')
   delfunc R
   exe bufnr .. "bw!"
 endfunc
 
+" 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
+
 " vim: shiftwidth=2 sts=2 expandtab