[nginx] Core: stricter UTF-8 handling in ngx_utf8_decode().

[Roman Arutyunyan]

details:   https://hg.nginx.org/nginx/rev/a10210a45c8b
branches:  
changeset: 8142:a10210a45c8b
user:      Yugo Horie <u5.horie at gmail.com>
date:      Thu Feb 23 08:09:50 2023 +0900
description:
Core: stricter UTF-8 handling in ngx_utf8_decode().

An UTF-8 octet sequence cannot start with a 11111xxx byte (above 0xf8),
see https://datatracker.ietf.org/doc/html/rfc3629#section-3.  Previously,
such bytes were accepted by ngx_utf8_decode() and misinterpreted as 11110xxx
bytes (as in a 4-byte sequence).  While unlikely, this can potentially cause
issues.

Fix is to explicitly reject such bytes in ngx_utf8_decode().

diffstat:

 src/core/ngx_string.c |  7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diffs (17 lines):

diff -r 2acb00b9b5ff -r a10210a45c8b src/core/ngx_string.c
--- a/src/core/ngx_string.c	Thu Feb 23 20:50:03 2023 +0300
+++ b/src/core/ngx_string.c	Thu Feb 23 08:09:50 2023 +0900
@@ -1364,7 +1364,12 @@ ngx_utf8_decode(u_char **p, size_t n)
 
     u = **p;
 
-    if (u >= 0xf0) {
+    if (u >= 0xf8) {
+
+        (*p)++;
+        return 0xffffffff;
+
+    } else if (u >= 0xf0) {
 
         u &= 0x07;
         valid = 0xffff;