vim-patch:9.0.1969: [security] buffer-overflow in trunc_string()

Problem:  buffer-overflow in trunc_string()
Solution: Add NULL at end of buffer

Currently trunc_string() assumes that when the string is too long,
buf[e-1] will always be writeable. But that assumption may not always be
true. The condition currently looks like this

    else if (e + 3 < buflen)
    [...]
    else
    {
	// can't fit in the "...", just truncate it
	buf[e - 1] = NUL;
    }

but this means, we may run into the last else clause with e still being
larger than buflen. So a buffer overflow occurs.

So instead of using `buf[e - 1]`, let's just always
truncate at `buf[buflen - 1]` which should always be writable.

3bd7fa12e1

vim-patch:9.0.2004: Missing test file

Problem:  Missing test file
Solution: git-add the file to the repo

closes: vim/vim#13305

d4afbdd071

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

src/nvim/message.c

@@ -468,7 +468,7 @@ void trunc_string(const char *s, char *buf, int room_in, int buflen)
     buf[e + 3 + len - 1] = NUL;
   } else {
     // can't fit in the "...", just truncate it
-    buf[e - 1] = NUL;
+    buf[buflen - 1] = NUL;
   }
 }
 

test/functional/legacy/messages_spec.lua

@@ -6,6 +6,7 @@ local exec = helpers.exec
 local feed = helpers.feed
 local meths = helpers.meths
 local nvim_dir = helpers.nvim_dir
+local assert_alive = helpers.assert_alive
 
 before_each(clear)
 
@@ -758,4 +759,9 @@ describe('messages', function()
     ]])
     os.remove('b.txt')
   end)
+
+  it('no crash when truncating overlong message', function()
+    pcall(command, 'source test/old/testdir/crash/vim_msg_trunc_poc')
+    assert_alive()
+  end)
 end)

test/old/testdir/test_crash.vim

@@ -66,6 +66,12 @@ func Test_crash1()
     \ '  && echo "crash 7: [OK]" >> X_crash1_result.txt' .. "\<cr>")
   call TermWait(buf, 3000)
 
+  let file = 'crash/vim_msg_trunc_poc'
+  let args = printf(cmn_args, vim, file)
+  call term_sendkeys(buf, args ..
+    \ '  || echo "crash 8: [OK]" >> X_crash1_result.txt' .. "\<cr>")
+  call TermWait(buf, 3000)
+
   " clean up
   exe buf .. "bw!"
 
@@ -79,6 +85,7 @@ func Test_crash1()
       \ 'crash 5: [OK]',
       \ 'crash 6: [OK]',
       \ 'crash 7: [OK]',
+      \ 'crash 8: [OK]',
       \ ]
 
   call assert_equal(expected, getline(1, '$'))