vim-patch:9.0.2121: [security]: use-after-free in ex_substitute

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

closes: vim/vim#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

26c11c5688

Co-authored-by: Christian Brabandt <cb@256bit.org>

src/nvim/ex_cmds.c

@@ -3367,8 +3367,9 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
 
     // Small incompatibility: vi sees '\n' as end of the command, but in
     // Vim we want to use '\n' to find/substitute a NUL.
-    sub = cmd;              // remember the start of the substitution
+    char *p = cmd;  // remember the start of the substitution
     cmd = skip_substitute(cmd, delimiter);
+    sub = xstrdup(p);
 
     if (!eap->skip && cmdpreview_ns <= 0) {
       sub_set_replacement((SubReplacementString) {
@@ -3383,7 +3384,7 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
       return 0;
     }
     pat = NULL;                 // search_regcomp() will use previous pattern
-    sub = old_sub.sub;
+    sub = xstrdup(old_sub.sub);
 
     // Vi compatibility quirk: repeating with ":s" keeps the cursor in the
     // last column after using "$".
@@ -3391,6 +3392,7 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
   }
 
   if (sub != NULL && sub_joining_lines(eap, pat, sub, cmd, cmdpreview_ns <= 0)) {
+    xfree(sub);
     return 0;
   }
 
@@ -3405,11 +3407,13 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
     i = getdigits_int(&cmd, true, INT_MAX);
     if (i <= 0 && !eap->skip && subflags.do_error) {
       emsg(_(e_zerocount));
+      xfree(sub);
       return 0;
     } else if (i >= INT_MAX) {
       char buf[20];
       vim_snprintf(buf, sizeof(buf), "%d", i);
       semsg(_(e_val_too_large), buf);
+      xfree(sub);
       return 0;
     }
     eap->line1 = eap->line2;
@@ -3425,17 +3429,20 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
     eap->nextcmd = check_nextcmd(cmd);
     if (eap->nextcmd == NULL) {
       semsg(_(e_trailing_arg), cmd);
+      xfree(sub);
       return 0;
     }
   }
 
   if (eap->skip) {          // not executing commands, only parsing
+    xfree(sub);
     return 0;
   }
 
   if (!subflags.do_count && !MODIFIABLE(curbuf)) {
     // Substitution is not allowed in non-'modifiable' buffer
     emsg(_(e_modifiable));
+    xfree(sub);
     return 0;
   }
 
@@ -3444,6 +3451,7 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
     if (subflags.do_error) {
       emsg(_(e_invcmd));
     }
+    xfree(sub);
     return 0;
   }
 
@@ -3458,22 +3466,20 @@ static int do_sub(exarg_T *eap, const proftime_T timeout, const int cmdpreview_n
 
   assert(sub != NULL);
 
-  char *sub_copy = NULL;
-
   // If the substitute pattern starts with "\=" then it's an expression.
   // Make a copy, a recursive function may free it.
   // Otherwise, '~' in the substitute pattern is replaced with the old
   // pattern.  We do it here once to avoid it to be replaced over and over
   // again.
   if (sub[0] == '\\' && sub[1] == '=') {
-    sub = xstrdup(sub);
-    sub_copy = sub;
+    char *p = xstrdup(sub);
+    xfree(sub);
+    sub = p;
   } else {
-    char *newsub = regtilde(sub, magic_isset(), cmdpreview_ns > 0);
-    if (newsub != sub) {
-      // newsub was allocated, free it later.
-      sub_copy = newsub;
-      sub = newsub;
+    char *p = regtilde(sub, magic_isset(), cmdpreview_ns > 0);
+    if (p != sub) {
+      xfree(sub);
+      sub = p;
     }
   }
 
@@ -4244,7 +4250,7 @@ skip:
   }
 
   vim_regfree(regmatch.regprog);
-  xfree(sub_copy);
+  xfree(sub);
 
   // Restore the flag values, they can be used for ":&&".
   subflags.do_all = save_do_all;

test/old/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()
@@ -1448,11 +1473,30 @@ 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