vim-patch:9.0.1840: [security] use-after-free in do_ecmd (#24993)

Problem:  use-after-free in do_ecmd
Solution: Verify oldwin pointer after reset_VIsual()

e1dc9a6275

N/A patches for version.c:
vim-patch:9.0.1841: style: trailing whitespace in ex_cmds.c

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

src/nvim/ex_cmds.c

@@ -2230,8 +2230,16 @@ int do_ecmd(int fnum, char *ffname, char *sfname, exarg_T *eap, linenr_T newlnum
 
   // End Visual mode before switching to another buffer, so the text can be
   // copied into the GUI selection buffer.
+  // Careful: may trigger ModeChanged() autocommand
+
+  // Should we block autocommands here?
   reset_VIsual();
 
+  // autocommands freed window :(
+  if (oldwin != NULL && !win_valid(oldwin)) {
+    oldwin = NULL;
+  }
+
   if ((command != NULL || newlnum > (linenr_T)0)
       && *get_vim_var_str(VV_SWAPCOMMAND) == NUL) {
     // Set v:swapcommand for the SwapExists autocommands.

test/functional/legacy/crash_spec.lua

@@ -0,0 +1,16 @@
+local helpers = require('test.functional.helpers')(after_each)
+local assert_alive = helpers.assert_alive
+local clear = helpers.clear
+local command = helpers.command
+local feed = helpers.feed
+
+before_each(clear)
+
+-- oldtest: Test_crash1()
+it('no crash when ending Visual mode while editing buffer closes window', function()
+  command('new')
+  command('autocmd ModeChanged v:n ++once close')
+  feed('v')
+  command('enew')
+  assert_alive()
+end)

test/old/testdir/test_crash.vim

@@ -0,0 +1,25 @@
+" Some tests, that used to crash Vim
+source check.vim
+source screendump.vim
+
+CheckScreendump
+
+func Test_crash1()
+  " The following used to crash Vim
+  let opts = #{wait_for_ruler: 0}
+  let args = ' -u NONE -i NONE -n -e -s -S '
+  let buf = RunVimInTerminal(args .. ' crash/poc_huaf1', opts)
+  call VerifyScreenDump(buf, 'Test_crash_01', {})
+  exe buf .. "bw!"
+
+  let buf = RunVimInTerminal(args .. ' crash/poc_huaf2', opts)
+  call VerifyScreenDump(buf, 'Test_crash_01', {})
+  exe buf .. "bw!"
+
+  let buf = RunVimInTerminal(args .. ' crash/poc_huaf3', opts)
+  call VerifyScreenDump(buf, 'Test_crash_01', {})
+  exe buf .. "bw!"
+
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab