[nginx] Fixed incorrect length handling in ngx_utf8_length().

[Maxim Dounin]

details:   https://hg.nginx.org/nginx/rev/a42a6dfeb01a
branches:  
changeset: 7494:a42a6dfeb01a
user:      Maxim Dounin <mdounin at mdounin.ru>
date:      Mon Apr 15 20:14:07 2019 +0300
description:
Fixed incorrect length handling in ngx_utf8_length().

Previously, ngx_utf8_decode() was called from ngx_utf8_length() with
incorrect length, potentially resulting in out-of-bounds read when
handling invalid UTF-8 strings.

In practice out-of-bounds reads are not possible though, as autoindex, the
only user of ngx_utf8_length(), provides null-terminated strings, and
ngx_utf8_decode() anyway returns an errors when it sees a null in the
middle of an UTF-8 sequence.

Reported by Yunbin Liu.

diffstat:

 src/core/ngx_string.c |  2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diffs (12 lines):

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1381,7 +1381,7 @@ ngx_utf8_length(u_char *p, size_t n)
             continue;
         }
 
-        if (ngx_utf8_decode(&p, n) > 0x10ffff) {
+        if (ngx_utf8_decode(&p, last - p) > 0x10ffff) {
             /* invalid UTF-8 */
             return n;
         }