Make pg_regexec() robust against out-of-range search_start.

tglsfdc · tglsfdc · commit 7e420072ea4a · 2021-09-11T15:19:49.000-04:00
If search_start is greater than the length of the string, we should just return REG_NOMATCH immediately. (Note that the equality case should *not* be rejected, since the pattern might be able to match zero characters.) This guards various internal assumptions that the min of a range of string positions is not more than the max. Violation of those assumptions could allow an attempt to fetch string[search_start-1], possibly causing a crash. Jaime Casanova pointed out that this situation is reachable with the new regexp_xxx functions that accept a user-specified start position. I don't believe it's reachable via any in-core call site in v14 and below. However, extensions could possibly call pg_regexec with an out-of-range search_start, so let's back-patch the fix anyway. Discussion: https://postgr.es/m/20210911180357.GA6870@ahch-to
diff --git a/src/backend/regex/regexec.c b/src/backend/regex/regexec.c
@@ -196,6 +196,8 @@ pg_regexec(regex_t *re,
 		return REG_INVARG;
 	if (re->re_csize != sizeof(chr))
 		return REG_MIXED;
+	if (search_start > len)
+		return REG_NOMATCH;
 
 	/* Initialize locale-dependent support */
 	pg_set_regex_collation(re->re_collation);
