bpf: Support dual-stack sockets in bpf_tcp_check_syncookie

bpf_tcp_gen_syncookie looks at the IP version in the IP header and
validates the address family of the socket. It supports IPv4 packets in
AF_INET6 dual-stack sockets.

On the other hand, bpf_tcp_check_syncookie looks only at the address
family of the socket, ignoring the real IP version in headers, and
validates only the packet size. This implementation has some drawbacks:

1. Packets are not validated properly, allowing a BPF program to trick
   bpf_tcp_check_syncookie into handling an IPv6 packet on an IPv4
   socket.

2. Dual-stack sockets fail the checks on IPv4 packets. IPv4 clients end
   up receiving a SYNACK with the cookie, but the following ACK gets
   dropped.

This patch fixes these issues by changing the checks in
bpf_tcp_check_syncookie to match the ones in bpf_tcp_gen_syncookie. IP
version from the header is taken into account, and it is validated
properly with address family.

Fixes: 399040847084 ("bpf: add helper to check for a valid SYN cookie")
Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Acked-by: Arthur Fabre <afabre@cloudflare.com>
Link: https://lore.kernel.org/bpf/20220406124113.2795730-1-maximmi@nvidia.com

diff --git a/net/core/filter.c b/net/core/filter.c
index a7044e98765ec5e5b55724527aa61068ccaec20a..64470a727ef77d25fe25fd38c861cac65fd3e95f 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7016,24 +7016,33 @@ BPF_CALL_5(bpf_tcp_check_syncookie, struct sock *, sk, void *, iph, u32, iph_len
        if (!th->ack || th->rst || th->syn)
                return -ENOENT;
 
+       if (unlikely(iph_len < sizeof(struct iphdr)))
+               return -EINVAL;
+
        if (tcp_synq_no_recent_overflow(sk))
                return -ENOENT;
 
        cookie = ntohl(th->ack_seq) - 1;
 
-       switch (sk->sk_family) {
-       case AF_INET:
-               if (unlikely(iph_len < sizeof(struct iphdr)))
+       /* Both struct iphdr and struct ipv6hdr have the version field at the
+        * same offset so we can cast to the shorter header (struct iphdr).
+        */
+       switch (((struct iphdr *)iph)->version) {
+       case 4:
+               if (sk->sk_family == AF_INET6 && ipv6_only_sock(sk))
                        return -EINVAL;
 
                ret = __cookie_v4_check((struct iphdr *)iph, th, cookie);
                break;
 
 #if IS_BUILTIN(CONFIG_IPV6)
-       case AF_INET6:
+       case 6:
                if (unlikely(iph_len < sizeof(struct ipv6hdr)))
                        return -EINVAL;
 
+               if (sk->sk_family != AF_INET6)
+                       return -EINVAL;
+
                ret = __cookie_v6_check((struct ipv6hdr *)iph, th, cookie);
                break;
 #endif /* CONFIG_IPV6 */