vim-patch:9.1.0648: [security] double-free in dialog_changed()

Problem:  [security] double-free in dialog_changed()
          (SuyueGuo)
Solution: Only clear pointer b_sfname pointer, if it is different
          than the b_ffname pointer.  Don't try to free b_fname,
          set it to NULL instead.

fixes: vim/vim#15403

Github Advisory:
https://github.com/vim/vim/security/advisories/GHSA-46pw-v7qw-xc2f

b29f4abcd4

Co-authored-by: Christian Brabandt <cb@256bit.org>
(cherry picked from commit 6967c08840)

src/nvim/ex_cmds2.c

@@ -226,9 +226,12 @@ void dialog_changed(buf_T *buf, bool checkall)
 
     // restore to empty when write failed
     if (empty_bufname) {
-      XFREE_CLEAR(buf->b_fname);
-      XFREE_CLEAR(buf->b_ffname);
+      // prevent double free
+      if (buf->b_sfname != buf->b_ffname) {
         XFREE_CLEAR(buf->b_sfname);
+      }
+      buf->b_fname = NULL;
+      XFREE_CLEAR(buf->b_ffname);
       unchanged(buf, true, false);
     }
   } else if (ret == VIM_NO) {

test/functional/legacy/crash_spec.lua

@@ -8,6 +8,7 @@ local eq = t.eq
 local eval = n.eval
 local exec = n.exec
 local feed = n.feed
+local pcall_err = t.pcall_err
 
 before_each(clear)
 
@@ -51,3 +52,14 @@ it('no crash when closing window with tag in loclist', function()
   eq(0, eval('bufexists(g:qf_bufnr)'))
   assert_alive()
 end)
+
+it('no crash when writing "Untitled" file fails', function()
+  t.mkdir('Untitled')
+  finally(function()
+    vim.uv.fs_rmdir('Untitled')
+  end)
+  feed('ifoobar')
+  command('set bufhidden=unload')
+  eq('Vim(enew):E502: "Untitled" is a directory', pcall_err(command, 'confirm enew'))
+  assert_alive()
+end)

test/old/testdir/test_crash.vim

@@ -196,6 +196,12 @@ func Test_crash1_3()
   call term_sendkeys(buf, args)
   call TermWait(buf, 50)
 
+  let file = 'crash/dialog_changed_uaf'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'\<cr>"
+  let args = printf(cmn_args, vim, file)
+  call term_sendkeys(buf, args)
+  call TermWait(buf, 150)
+
   " clean up
   exe buf .. "bw!"
   bw!