[PATCH] Variables: avoid possible buffer overrun with some "$sent_http_*"

[Sergey Kandaurov]

# HG changeset patch
# User Sergey Kandaurov <pluknet at nginx.com>
# Date 1682617947 -14400
#      Thu Apr 27 21:52:27 2023 +0400
# Node ID c1ec385d885fba38f15d54263944eed2c74b5733
# Parent  77d5c662f3d9d9b90425128109d3369c30ef5f07
Variables: avoid possible buffer overrun with some "$sent_http_*".

The existing logic to evaluate multi header "$sent_http_*" variables,
such as $sent_http_cache_control, as previously introduced in 1.23.0,
doesn't take into account that one or more elements can be cleared,
yet still present in a linked list, pointed to by the next field.
Such elements don't contribute to the resulting variable length, an
attempt to append a separator for them ends up in out of bounds write.

This is not possible with standard modules, though at least one third
party module is known to override multi header values this way, so it
makes sense to harden the logic.

The fix restores a generic boundary check.

diff --git a/src/http/ngx_http_variables.c b/src/http/ngx_http_variables.c
--- a/src/http/ngx_http_variables.c
+++ b/src/http/ngx_http_variables.c
@@ -828,7 +828,7 @@ ngx_http_variable_headers_internal(ngx_h
     ngx_http_variable_value_t *v, uintptr_t data, u_char sep)
 {
     size_t            len;
-    u_char           *p;
+    u_char           *p, *end;
     ngx_table_elt_t  *h, *th;
 
     h = *(ngx_table_elt_t **) ((char *) r + data);
@@ -870,6 +870,8 @@ ngx_http_variable_headers_internal(ngx_h
     v->len = len;
     v->data = p;
 
+    end = p + len;
+
     for (th = h; th; th = th->next) {
 
         if (th->hash == 0) {
@@ -878,7 +880,7 @@ ngx_http_variable_headers_internal(ngx_h
 
         p = ngx_copy(p, th->value.data, th->value.len);
 
-        if (th->next == NULL) {
+        if (p == end) {
             break;
         }