Variables: avoid possible buffer overrun with some "$sent_http_*".HEADmasterbranches/default

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.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/http/ngx_http_variables.c b/src/http/ngx_http_variables.c
index 16ffda3fe..4f0bd0e4b 100644
--- a/src/http/ngx_http_variables.c
+++ b/src/http/ngx_http_variables.c
@@ -828,7 +828,7 @@ ngx_http_variable_headers_internal(ngx_http_request_t *r,
     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_http_request_t *r,
     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_http_request_t *r,
 
         p = ngx_copy(p, th->value.data, th->value.len);
 
-        if (th->next == NULL) {
+        if (p == end) {
             break;
         }