vim-patch:9.0.2140: [security]: use-after-free in win-enter

Problem:  [security]: use-after-free in win-enter
Solution: validate window pointer before calling win_enter()

win_goto() may stop visual mode, if it is active. However, this may in
turn trigger the ModeChanged autocommand, which could potentially free
the wp pointer which was valid before now became stale and points to now
freed memory.

So before calling win_enter(), let's verify one more time, that the
wp pointer still points to a valid window structure.

Reported by @henices, thanks!

eec0c2b3a4

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

src/nvim/window.c

@@ -4470,11 +4470,12 @@ void tabpage_move(int nr)
   redraw_tabline = true;
 }
 
-// Go to another window.
-// When jumping to another buffer, stop Visual mode.  Do this before
-// changing windows so we can yank the selection into the '*' register.
-// When jumping to another window on the same buffer, adjust its cursor
-// position to keep the same Visual area.
+/// Go to another window.
+/// When jumping to another buffer, stop Visual mode.  Do this before
+/// changing windows so we can yank the selection into the '*' register.
+/// (note: this may trigger ModeChanged autocommand!)
+/// When jumping to another window on the same buffer, adjust its cursor
+/// position to keep the same Visual area.
 void win_goto(win_T *wp)
 {
   win_T *owp = curwin;
@@ -4485,11 +4486,17 @@ void win_goto(win_T *wp)
   }
 
   if (wp->w_buffer != curbuf) {
+    // careful: triggers ModeChanged autocommand
     reset_VIsual_and_resel();
   } else if (VIsual_active) {
     wp->w_cursor = curwin->w_cursor;
   }
 
+  // autocommand may have made wp invalid
+  if (!win_valid(wp)) {
+    return;
+  }
+
   win_enter(wp, true);
 
   // Conceal cursor line in previous window, unconceal in current window.

test/functional/legacy/crash_spec.lua

@@ -6,7 +6,6 @@ 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')
@@ -14,3 +13,11 @@ it('no crash when ending Visual mode while editing buffer closes window', functi
   command('enew')
   assert_alive()
 end)
+
+it('no crash when ending Visual mode close the window to switch to', function()
+  command('new')
+  command('autocmd ModeChanged v:n ++once only')
+  feed('v')
+  command('wincmd p')
+  assert_alive()
+end)

test/old/testdir/test_crash.vim

@@ -128,6 +128,13 @@ func Test_crash1_2()
     \ '  && echo "crash 1: [OK]" > '.. result .. "\<cr>")
   call TermWait(buf, 150)
 
+  let file = 'crash/poc_win_enter_ext'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'"
+  let args = printf(cmn_args, vim, file)
+  call term_sendkeys(buf, args ..
+    \ '  && echo "crash 2: [OK]" >> '.. result .. "\<cr>")
+  call TermWait(buf, 350)
+
   " clean up
   exe buf .. "bw!"
 
@@ -135,6 +142,7 @@ func Test_crash1_2()
 
   let expected = [
       \ 'crash 1: [OK]',
+      \ 'crash 2: [OK]',
       \ ]
 
   call assert_equal(expected, getline(1, '$'))