vim-patch:9.1.0254: [security]: Heap buffer overflow when calling complete_add() in 'cfu'

Problem:  [security]: Heap buffer overflow when calling complete_add()
          in the first call of 'completefunc'
Solution: Call check_cursor() after calling 'completefunc' (zeertzjq)

closes: vim/vim#14391

0a419e07a7

src/nvim/insexpand.c

@@ -2435,7 +2435,8 @@ static void expand_by_function(int type, char *base)
   }
   textlock--;
 
-  curwin->w_cursor = pos;       // restore the cursor position
+  curwin->w_cursor = pos;  // restore the cursor position
+  check_cursor(curwin);  // make sure cursor position is valid, just in case
   validate_cursor(curwin);
   if (!equalpos(curwin->w_cursor, pos)) {
     emsg(_(e_compldel));
@@ -4059,6 +4060,7 @@ static int get_userdefined_compl_info(colnr_T curs_col)
 
   State = save_State;
   curwin->w_cursor = pos;  // restore the cursor position
+  check_cursor(curwin);  // make sure cursor position is valid, just in case
   validate_cursor(curwin);
   if (!equalpos(curwin->w_cursor, pos)) {
     emsg(_(e_compldel));

test/old/testdir/test_ins_complete.vim

@@ -2490,4 +2490,26 @@ func Test_complete_changed_complete_info()
   call StopVimInTerminal(buf)
 endfunc
 
+func Test_completefunc_first_call_complete_add()
+  new
+
+  func Complete(findstart, base) abort
+    if a:findstart
+      let col = col('.')
+      call complete_add('#')
+      return col - 1
+    else
+      return []
+    endif
+  endfunc
+
+  set completeopt=longest completefunc=Complete
+  " This used to cause heap-buffer-overflow
+  call assert_fails('call feedkeys("ifoo#\<C-X>\<C-U>", "xt")', 'E840:')
+
+  delfunc Complete
+  set completeopt& completefunc&
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable