From: Ozgur Karatas <ozgur@goosey.org>

flowi4_init_output() function returns an argument and if opt->srr is
true and opt->faddr is assigned to be checked before opt->faddr is
used but if opt->srr seems to be true and opt->faddr is not set
properly yet.

opt itself will be an incompletely initialized struct and this access
may cause a crash.
* added daddr
* like readability by passing a single daddr argument to
flowi4_init_output() call.

Signed-off-by: Ozgur Karatas <ozgur@goosey.org>

---
 net/ipv4/syncookies.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/syncookies.c b/net/ipv4/syncookies.c
index 5459a78b9809..2ff92d512825 100644
--- a/net/ipv4/syncookies.c
+++ b/net/ipv4/syncookies.c
@@ -408,6 +408,7 @@ struct sock *cookie_v4_check(struct sock *sk,
struct sk_buff *skb)
        struct flowi4 fl4;
        struct rtable *rt;
        __u8 rcv_wscale;
+       __be32 daddr;
        int full_space;
        SKB_DR(reason);

@@ -442,6 +443,17 @@ struct sock *cookie_v4_check(struct sock *sk,
struct sk_buff *skb)
                goto out_free;
        }

+        /* Safely determine destination address considered SRR option.
+         * The flowi4 destination address is derived from opt->faddr
if opt->srr is set.
+         * However IP options are not always present in the skb and
accessing opt->faddr
+         * without validating opt->optlen and opt->srr can lead to
undefined behavior.
+         */
+        if (opt && opt->optlen && opt->srr) {
+                daddr = opt->faddr;
+        } else {
+                daddr = ireq->ir_rmt_addr;
+        }
+
        tcp_ao_syncookie(sk, skb, req, AF_INET);

        /*
@@ -453,7 +465,7 @@ struct sock *cookie_v4_check(struct sock *sk,
struct sk_buff *skb)
        flowi4_init_output(&fl4, ireq->ir_iif, ireq->ir_mark,
                           ip_sock_rt_tos(sk), ip_sock_rt_scope(sk),
                           IPPROTO_TCP, inet_sk_flowi_flags(sk),
-                          opt->srr ? opt->faddr : ireq->ir_rmt_addr,
+                          daddr,
                           ireq->ir_loc_addr, th->source, th->dest, sk->sk_uid);
        security_req_classify_flow(req, flowi4_to_flowi_common(&fl4));
        rt = ip_route_output_key(net, &fl4);
--
2.39.5