fix(terminal): handle C0 characters in OSC terminator (#30090)

When a C0 character is present in an OSC terminator (i.e. after the ESC
but before a \ (0x5c) or printable character), vterm executes the
control character and resets the current string fragment. If the C0
character is the final byte in the sequence, the string fragment has a
zero length. However, because the VT parser is still in the "escape"
state, vterm attempts to subtract 1 from the string length (to account
for the escape character). When the string fragment is empty, this
causes an underflow in the unsigned size variable, resulting in a buffer
overflow.

The fix is simple: explicitly check if the string length is non-zero
before subtracting.

src/nvim/terminal.c

@@ -271,10 +271,11 @@ static int parse_osc8(VTermStringFragment frag, int *attr)
 }
 
 static int on_osc(int command, VTermStringFragment frag, void *user)
+  FUNC_ATTR_NONNULL_ALL
 {
   Terminal *term = user;
 
-  if (frag.str == NULL) {
+  if (frag.str == NULL || frag.len == 0) {
     return 0;
   }
 

src/vterm/parser.c

@@ -1,5 +1,6 @@
 #include "vterm_internal.h"
 
+#include <assert.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -190,8 +191,10 @@ size_t vterm_input_write(VTerm *vt, const char *bytes, size_t len)
           ((!IS_STRING_STATE() || c == 0x5c))) {
         c += 0x40;
         c1_allowed = true;
-        if(string_len)
+        if(string_len) {
+          assert(string_len > 0);
           string_len -= 1;
+        }
         vt->parser.in_esc = false;
       }
       else {
@@ -377,9 +380,12 @@ string_state:
 
   if(string_start) {
     size_t string_len = bytes + pos - string_start;
-    if(vt->parser.in_esc)
-      string_len -= 1;
-    string_fragment(vt, string_start, string_len, false);
+    if (string_len > 0) {
+      if(vt->parser.in_esc) {
+        string_len -= 1;
+      }
+      string_fragment(vt, string_start, string_len, false);
+    }
   }
 
   return len;

test/functional/terminal/parser_spec.lua

@@ -0,0 +1,15 @@
+local n = require('test.functional.testnvim')()
+
+local clear = n.clear
+local api = n.api
+local assert_alive = n.assert_alive
+
+describe(':terminal', function()
+  before_each(clear)
+
+  it('handles invalid OSC terminators #30084', function()
+    local chan = api.nvim_open_term(0, {})
+    api.nvim_chan_send(chan, '\027]8;;https://example.com\027\\Example\027]8;;\027\n')
+    assert_alive()
+  end)
+end)