PCRE: Only remember valid UTF-8 if start offset zero

PCRE only validates the string starting from the start offset
(minus maximum look-behind, but let's ignore that), so we can
only remember that the string is fully valid UTF-8 is the original
start offset is zero.

Author: nikic

NEWS

@@ -13,6 +13,7 @@ PHP                                                                        NEWS
 - PCRE:
   . Fixed bug #79188 (Memory corruption in preg_replace/preg_replace_callback
     and unicode). (Nikita)
+  . Fixed bug #79241 (Segmentation fault on preg_match()). (Nikita)
 
 ?? ??? ????, PHP 7.4.3
 

ext/pcre/php_pcre.c

@@ -1167,7 +1167,7 @@ PHPAPI void php_pcre_match_impl(pcre_cache_entry *pce, zend_string *subject_str,
 	PCRE2_SPTR       mark = NULL;		/* Target for MARK name */
 	zval			 marks;				/* Array of marks for PREG_PATTERN_ORDER */
 	pcre2_match_data *match_data;
-	PCRE2_SIZE		 start_offset2;
+	PCRE2_SIZE		 start_offset2, orig_start_offset;
 
 	char *subject = ZSTR_VAL(subject_str);
 	size_t subject_len = ZSTR_LEN(subject_str);
@@ -1263,8 +1263,10 @@ PHPAPI void php_pcre_match_impl(pcre_cache_entry *pce, zend_string *subject_str,
 		}
 	}
 
-	options = (pce->compile_options & PCRE2_UTF) && !is_known_valid_utf8(subject_str, start_offset2)
-		? 0 : PCRE2_NO_UTF_CHECK;
+	orig_start_offset = start_offset2;
+	options =
+		(pce->compile_options & PCRE2_UTF) && !is_known_valid_utf8(subject_str, orig_start_offset)
+			? 0 : PCRE2_NO_UTF_CHECK;
 
 	/* Execute the regular expression. */
 #ifdef HAVE_PCRE_JIT_SUPPORT
@@ -1454,7 +1456,8 @@ PHPAPI void php_pcre_match_impl(pcre_cache_entry *pce, zend_string *subject_str,
 
 	if (PCRE_G(error_code) == PHP_PCRE_NO_ERROR) {
 		/* If there was no error and we're in /u mode, remember that the string is valid UTF-8. */
-		if ((pce->compile_options & PCRE2_UTF) && !ZSTR_IS_INTERNED(subject_str)) {
+		if ((pce->compile_options & PCRE2_UTF)
+				&& !ZSTR_IS_INTERNED(subject_str) && orig_start_offset == 0) {
 			GC_ADD_FLAGS(subject_str, IS_STR_VALID_UTF8);
 		}
 

ext/pcre/tests/bug79241.phpt

@@ -15,8 +15,19 @@ var_dump(preg_match($pattern, $text, $matches, 0, 0));
 var_dump(preg_match($pattern, $text, $matches, 0, 1));
 var_dump(preg_last_error() == PREG_BAD_UTF8_OFFSET_ERROR);
 
+echo "\n";
+
+$text = "VA\xff"; $text .= "LID";
+var_dump(preg_match($pattern, $text, $matches, 0, 4));
+var_dump(preg_match($pattern, $text, $matches, 0, 0));
+var_dump(preg_last_error() == PREG_BAD_UTF8_ERROR);
+
 ?>
 --EXPECT--
 int(0)
 bool(false)
 bool(true)
+
+int(1)
+bool(false)
+bool(true)