vim-patch:9.1.2128: Heap use after free in buf_check_timestamp()

Problem:  heap UAF if autocommands from reloading a file changed outside
          of Vim wipe its buffer.
Solution: Validate the bufref after buf_reload (Sean Dewar)

closes: vim/vim#19317

392b428d12

Co-authored-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
(cherry picked from commit fede568692)

src/nvim/fileio.c

@@ -3060,7 +3060,7 @@ int buf_check_timestamp(buf_T *buf)
   if (reload != RELOAD_NONE) {
     // Reload the buffer.
     buf_reload(buf, orig_mode, reload == RELOAD_DETECT);
-    if (buf->b_p_udf && buf->b_ffname != NULL) {
+    if (bufref_valid(&bufref) && buf->b_p_udf && buf->b_ffname != NULL) {
       uint8_t hash[UNDO_HASH_SIZE];
 
       // Any existing undo file is unusable, write it now.

test/old/testdir/test_filechanged.vim

@@ -289,4 +289,27 @@ func Test_FileChangedShell_newbuf()
   call delete('Xfile')
 endfunc
 
+func Test_file_changed_wipeout()
+  call writefile(['foo'], 'Xchanged_bw', 'D')
+  edit Xchanged_bw
+  augroup FileChangedWipeout
+    autocmd FileChangedShell * ++once let v:fcs_choice = 'reload'
+    autocmd BufReadPost * ++once %bw!
+  augroup END
+
+  " Need to wait until the timestamp would change.
+  if has('nanotime')
+    sleep 10m
+  else
+    sleep 2
+  endif
+  call writefile(['bar'], 'Xchanged_bw')
+  call assert_equal(1, bufexists('Xchanged_bw'))
+  checktime " used to be a heap UAF
+  call assert_equal(0, bufexists('Xchanged_bw'))
+
+  au! FileChangedWipeout
+  %bw!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab